Internet DRAFT - draft-ietf-cose-msg
draft-ietf-cose-msg
COSE Working Group J. Schaad
Internet-Draft August Cellars
Intended status: Standards Track November 22, 2016
Expires: May 26, 2017
CBOR Object Signing and Encryption (COSE)
draft-ietf-cose-msg-24
Abstract
Concise Binary Object Representation (CBOR) is data format designed
for small code size and small message size. There is a need for the
ability to have basic security services defined for this data format.
This document defines the CBOR Object Signing and Encryption (COSE)
specification. This specification describes how to create and
process signature, message authentication codes and encryption using
CBOR for serialization. This specification additionally specifies
how to represent cryptographic keys using CBOR.
Contributing to this document
The source for this draft is being maintained in GitHub. Suggested
changes should be submitted as pull requests at <https://github.com/
cose-wg/cose-spec>. Instructions are on that page as well.
Editorial changes can be managed in GitHub, but any substantial
issues need to be discussed on the COSE mailing list.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 26, 2017.
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Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
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to this document. Code Components extracted from this document must
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Design changes from JOSE . . . . . . . . . . . . . . . . 5
1.2. Requirements Terminology . . . . . . . . . . . . . . . . 6
1.3. CBOR Grammar . . . . . . . . . . . . . . . . . . . . . . 6
1.4. CBOR Related Terminology . . . . . . . . . . . . . . . . 7
1.5. Document Terminology . . . . . . . . . . . . . . . . . . 8
2. Basic COSE Structure . . . . . . . . . . . . . . . . . . . . 8
3. Header Parameters . . . . . . . . . . . . . . . . . . . . . . 10
3.1. Common COSE Headers Parameters . . . . . . . . . . . . . 12
4. Signing Objects . . . . . . . . . . . . . . . . . . . . . . . 16
4.1. Signing with One or More Signers . . . . . . . . . . . . 16
4.2. Signing with One Signer . . . . . . . . . . . . . . . . . 18
4.3. Externally Supplied Data . . . . . . . . . . . . . . . . 19
4.4. Signing and Verification Process . . . . . . . . . . . . 20
4.5. Computing Counter Signatures . . . . . . . . . . . . . . 22
5. Encryption Objects . . . . . . . . . . . . . . . . . . . . . 22
5.1. Enveloped COSE Structure . . . . . . . . . . . . . . . . 22
5.1.1. Content Key Distribution Methods . . . . . . . . . . 24
5.2. Single Recipient Encrypted . . . . . . . . . . . . . . . 25
5.3. How to encrypt and decrypt for AEAD Algorithms . . . . . 25
5.4. How to encrypt and decrypt for AE Algorithms . . . . . . 28
6. MAC Objects . . . . . . . . . . . . . . . . . . . . . . . . . 29
6.1. MACed Message with Recipients . . . . . . . . . . . . . . 30
6.2. MACed Messages with Implicit Key . . . . . . . . . . . . 31
6.3. How to compute and verify a MAC . . . . . . . . . . . . . 31
7. Key Objects . . . . . . . . . . . . . . . . . . . . . . . . . 33
7.1. COSE Key Common Parameters . . . . . . . . . . . . . . . 33
8. Signature Algorithms . . . . . . . . . . . . . . . . . . . . 36
8.1. ECDSA . . . . . . . . . . . . . . . . . . . . . . . . . . 37
8.1.1. Security Considerations . . . . . . . . . . . . . . . 39
8.2. Edwards-curve Digital Signature Algorithms (EdDSA) . . . 40
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8.2.1. Security Considerations . . . . . . . . . . . . . . . 41
9. Message Authentication (MAC) Algorithms . . . . . . . . . . . 41
9.1. Hash-based Message Authentication Codes (HMAC) . . . . . 41
9.1.1. Security Considerations . . . . . . . . . . . . . . . 43
9.2. AES Message Authentication Code (AES-CBC-MAC) . . . . . . 43
9.2.1. Security Considerations . . . . . . . . . . . . . . . 44
10. Content Encryption Algorithms . . . . . . . . . . . . . . . . 45
10.1. AES GCM . . . . . . . . . . . . . . . . . . . . . . . . 45
10.1.1. Security Considerations . . . . . . . . . . . . . . 46
10.2. AES CCM . . . . . . . . . . . . . . . . . . . . . . . . 47
10.2.1. Security Considerations . . . . . . . . . . . . . . 50
10.3. ChaCha20 and Poly1305 . . . . . . . . . . . . . . . . . 50
10.3.1. Security Considerations . . . . . . . . . . . . . . 51
11. Key Derivation Functions (KDF) . . . . . . . . . . . . . . . 51
11.1. HMAC-based Extract-and-Expand Key Derivation Function
(HKDF) . . . . . . . . . . . . . . . . . . . . . . . . . 52
11.2. Context Information Structure . . . . . . . . . . . . . 54
12. Content Key Distribution Methods . . . . . . . . . . . . . . 59
12.1. Direct Encryption . . . . . . . . . . . . . . . . . . . 59
12.1.1. Direct Key . . . . . . . . . . . . . . . . . . . . . 60
12.1.2. Direct Key with KDF . . . . . . . . . . . . . . . . 60
12.2. Key Wrapping . . . . . . . . . . . . . . . . . . . . . . 62
12.2.1. AES Key Wrapping . . . . . . . . . . . . . . . . . . 63
12.3. Key Transport . . . . . . . . . . . . . . . . . . . . . 64
12.4. Direct Key Agreement . . . . . . . . . . . . . . . . . . 64
12.4.1. ECDH . . . . . . . . . . . . . . . . . . . . . . . . 65
12.4.2. Security Considerations . . . . . . . . . . . . . . 69
12.5. Key Agreement with Key Wrap . . . . . . . . . . . . . . 69
12.5.1. ECDH . . . . . . . . . . . . . . . . . . . . . . . . 69
13. Key Object Parameters . . . . . . . . . . . . . . . . . . . . 71
13.1. Elliptic Curve Keys . . . . . . . . . . . . . . . . . . 72
13.1.1. Double Coordinate Curves . . . . . . . . . . . . . . 72
13.2. Octet Key Pair . . . . . . . . . . . . . . . . . . . . . 73
13.3. Symmetric Keys . . . . . . . . . . . . . . . . . . . . . 74
14. CBOR Encoder Restrictions . . . . . . . . . . . . . . . . . . 75
15. Application Profiling Considerations . . . . . . . . . . . . 75
16. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 77
16.1. CBOR Tag assignment . . . . . . . . . . . . . . . . . . 77
16.2. COSE Header Parameters Registry . . . . . . . . . . . . 77
16.3. COSE Header Algorithm Parameters Registry . . . . . . . 78
16.4. COSE Algorithms Registry . . . . . . . . . . . . . . . . 78
16.5. COSE Key Common Parameters Registry . . . . . . . . . . 79
16.6. COSE Key Type Parameters Registry . . . . . . . . . . . 80
16.7. COSE Key Type Registry . . . . . . . . . . . . . . . . . 81
16.8. COSE Elliptic Curve Parameters Registry . . . . . . . . 81
16.9. Media Type Registrations . . . . . . . . . . . . . . . . 82
16.9.1. COSE Security Message . . . . . . . . . . . . . . . 82
16.9.2. COSE Key media type . . . . . . . . . . . . . . . . 83
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16.10. CoAP Content-Format Registrations . . . . . . . . . . . 85
16.11. Expert Review Instructions . . . . . . . . . . . . . . . 86
17. Implementation Status . . . . . . . . . . . . . . . . . . . . 87
17.1. Author's Versions . . . . . . . . . . . . . . . . . . . 88
17.2. COSE Testing Library . . . . . . . . . . . . . . . . . . 88
18. Security Considerations . . . . . . . . . . . . . . . . . . . 89
19. References . . . . . . . . . . . . . . . . . . . . . . . . . 91
19.1. Normative References . . . . . . . . . . . . . . . . . . 91
19.2. Informative References . . . . . . . . . . . . . . . . . 92
Appendix A. Guidelines for External Data Authentication of
Algorithms . . . . . . . . . . . . . . . . . . . . . 95
A.1. Algorithm Identification . . . . . . . . . . . . . . . . 95
A.2. Counter Signature Without Headers . . . . . . . . . . . . 98
Appendix B. Two Layers of Recipient Information . . . . . . . . 99
Appendix C. Examples . . . . . . . . . . . . . . . . . . . . . . 101
C.1. Examples of Signed Message . . . . . . . . . . . . . . . 102
C.1.1. Single Signature . . . . . . . . . . . . . . . . . . 102
C.1.2. Multiple Signers . . . . . . . . . . . . . . . . . . 103
C.1.3. Counter Signature . . . . . . . . . . . . . . . . . . 104
C.1.4. Signature w/ Criticality . . . . . . . . . . . . . . 105
C.2. Single Signer Examples . . . . . . . . . . . . . . . . . 106
C.2.1. Single ECDSA signature . . . . . . . . . . . . . . . 106
C.3. Examples of Enveloped Messages . . . . . . . . . . . . . 107
C.3.1. Direct ECDH . . . . . . . . . . . . . . . . . . . . . 107
C.3.2. Direct plus Key Derivation . . . . . . . . . . . . . 108
C.3.3. Counter Signature on Encrypted Content . . . . . . . 109
C.3.4. Encrypted Content with External Data . . . . . . . . 111
C.4. Examples of Encrypted Messages . . . . . . . . . . . . . 111
C.4.1. Simple Encrypted Message . . . . . . . . . . . . . . 111
C.4.2. Encrypted Message w/ a Partial IV . . . . . . . . . . 112
C.5. Examples of MACed messages . . . . . . . . . . . . . . . 112
C.5.1. Shared Secret Direct MAC . . . . . . . . . . . . . . 112
C.5.2. ECDH Direct MAC . . . . . . . . . . . . . . . . . . . 113
C.5.3. Wrapped MAC . . . . . . . . . . . . . . . . . . . . . 114
C.5.4. Multi-recipient MACed message . . . . . . . . . . . . 115
C.6. Examples of MAC0 messages . . . . . . . . . . . . . . . . 116
C.6.1. Shared Secret Direct MAC . . . . . . . . . . . . . . 116
C.7. COSE Keys . . . . . . . . . . . . . . . . . . . . . . . . 117
C.7.1. Public Keys . . . . . . . . . . . . . . . . . . . . . 117
C.7.2. Private Keys . . . . . . . . . . . . . . . . . . . . 118
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 120
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 121
1. Introduction
There has been an increased focus on small, constrained devices that
make up the Internet of Things (IoT). One of the standards that has
come out of this process is the Concise Binary Object Representation
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(CBOR) [RFC7049]. CBOR extended the data model of the JavaScript
Object Notation (JSON) [RFC7159] by allowing for binary data, among
other changes. CBOR is being adopted by several of the IETF working
groups dealing with the IoT world as their encoding of data
structures. CBOR was designed specifically to be both small in terms
of messages transport and implementation size, as well having a
schema free decoder. A need exists to provide message security
services for IoT, and using CBOR as the message encoding format makes
sense.
The JOSE working group produced a set of documents
[RFC7515][RFC7516][RFC7517][RFC7518] using JSON that specified how to
process encryption, signatures and Message Authentication Code (MAC)
operations, and how to encode keys using JSON. This document defines
the CBOR Object Encryption and Signing (COSE) standard which does the
same thing for the CBOR encoding format. While there is a strong
attempt to keep the flavor of the original JOSE documents, two
considerations are taken into account:
o CBOR has capabilities that are not present in JSON and are
appropriate to use. One example of this is the fact that CBOR has
a method of encoding binary directly without first converting it
into a base64 encoded string.
o COSE is not a direct copy of the JOSE specification. In the
process of creating COSE, decisions that were made for JOSE were
re-examined. In many cases different results were decided on as
the criteria was not always the same.
1.1. Design changes from JOSE
o Define a single top message structure so that encrypted, signed
and MACed messages can easily be identified and still have a
consistent view.
o Signed messages distinguish between the protected and unprotected
parameters that relate to the content from those that relate to
the signature.
o MACed messages are separated from signed messages.
o MACed messages have the ability to use the same set of recipient
algorithms as enveloped messages for obtaining the MAC
authentication key.
o Use binary encodings for binary data rather than base64url
encodings.
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o Combine the authentication tag for encryption algorithms with the
cipher text.
o The set of cryptographic algorithms has been expanded in some
directions, and trimmed in others.
1.2. Requirements Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
When the words appear in lower case, their natural language meaning
is used.
1.3. CBOR Grammar
There is currently no standard CBOR grammar available for use by
specifications. The CBOR structures are therefore described in
prose.
The document was developed by first working on the grammar and then
developing the prose to go with it. An artifact of this is that the
prose was written using the primitive type strings defined by CBOR
Data Definition Language (CDDL) [I-D.greevenbosch-appsawg-cbor-cddl].
In this specification, the following primitive types are used:
any - non-specific value that permits all CBOR values to be placed
here.
bool - a boolean value (true: major type 7, value 21; false: major
type 7, value 20).
bstr - byte string (major type 2).
int - an unsigned integer or a negative integer.
nil - a null value (major type 7, value 22).
nint - a negative integer (major type 1).
tstr - a UTF-8 text string (major type 3).
uint - an unsigned integer (major type 0).
Two syntaxes from CDDL appear in this document as shorthand. These
are:
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FOO / BAR - indicates that either FOO or BAR can appear here
[+ FOO] - indicates that the type FOO appears one or more times in
an array
As well as the prose description, a version of a CBOR grammar is
presented in CDDL. Since CDDL has not been published as an RFC, this
grammar may not work with the final version of CDDL. The CDDL
grammar is informational, the prose description is normative.
The collected CDDL can be extracted from the XML version of this
document via the following XPath expression below. (Depending on the
XPath evaluator one is using, it may be necessary to deal with >
as an entity.)
//artwork[@type='CDDL']/text()
CDDL expects the initial non-terminal symbol to be the first symbol
in the file. For this reason the first fragment of CDDL is presented
here.
start = COSE_Messages / COSE_Key / COSE_KeySet / Internal_Types
; This is defined to make the tool quieter:
Internal_Types = Sig_structure / Enc_structure / MAC_structure /
COSE_KDF_Context
The non-terminal Internal_Types is defined for dealing with the
automated validation tools used during the writing of this document.
It references those non-terminals that are used for security
computations, but are not emitted for transport.
1.4. CBOR Related Terminology
In JSON, maps are called objects and only have one kind of map key: a
string. In COSE, we use strings, negative integers and unsigned
integers as map keys. The integers are used for compactness of
encoding and easy comparison. The inclusion of strings allows for an
additional range of short encoded values to be used as well. Since
the word "key" is mainly used in its other meaning, as a
cryptographic key, we use the term "label" for this usage as a map
key.
The presence of a label in a COSE map which is not a string or an
integer is an error. Applications can either fail processing or
process messages with incorrect labels, however they MUST NOT create
messages with incorrect labels.
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A CDDL grammar fragment is defined that defines the non-terminals
'label', as in the previous paragraph and 'values', which permits any
value to be used.
label = int / tstr
values = any
1.5. Document Terminology
In this document, we use the following terminology:
Byte is a synonym for octet.
Constrained Application Protocol (CoAP) is a specialized web transfer
protocol for use in constrained systems. It is defined in [RFC7252].
Authenticated Encryption (AE) [RFC5116] algorithms are those
encryption algorithms which provide an authentication check of the
contents algorithm with the encryption service.
Authenticated Encryption with Authenticated Data (AEAD) [RFC5116]
algorithms provide the same content authentication service as AE
algorithms, but additionally provide for authentication of non-
encrypted data as well.
2. Basic COSE Structure
The COSE object structure is designed so that there can be a large
amount of common code when parsing and processing the different types
of security messages. All of the message structures are built on the
CBOR array type. The first three elements of the array always
contain the same information:
1. The set of protected header parameters wrapped in a bstr.
2. The set of unprotected header parameters as a map.
3. The content of the message. The content is either the plain text
or the cipher text as appropriate. The content may be detached,
but the location is still used. The content is wrapped in a bstr
when present and is a nil value when detached.
Elements after this point are dependent on the specific message type.
COSE messages are also built using the concept of layers to separate
different types of cryptographic concepts. As an example of how this
works, consider the COSE_Encrypt message (Section 5.1). This message
type is broken into two layers: the content layer and the recipient
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layer. In the content layer, the plain text is encrypted and
information about the encrypted message are placed. In the recipient
layer, the content encryption key (CEK) is encrypted and information
about how it is encrypted for each recipient is placed. A single
layer version of the encryption message COSE_Encrypt0 (Section 5.2)
is provided for cases where the CEK is pre-shared.
Identification of which type of message has been presented is done by
the following methods:
1. The specific message type is known from the context. This may be
defined by a marker in the containing structure or by
restrictions specified by the application protocol.
2. The message type is identified by a CBOR tag. Messages with a
CBOR tag are known in this specification as tagged messages,
while those without the CBOR tag are known as untagged messages.
This document defines a CBOR tag for each of the message
structures. These tags can be found in Table 1.
3. When a COSE object is carried in a media type of application/
cose, the optional parameter 'cose-type' can be used to identify
the embedded object. The parameter is OPTIONAL if the tagged
version of the structure is used. The parameter is REQUIRED if
the untagged version of the structure is used. The value to use
with the parameter for each of the structures can be found in
Table 1.
4. When a COSE object is carried as a CoAP payload, the CoAP
Content-Format Option can be used to identify the message
content. The CoAP Content-Format values can be found in
Table 26. The CBOR tag for the message structure is not required
as each security message is uniquely identified.
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+-------+---------------+---------------+---------------------------+
| CBOR | cose-type | Data Item | Semantics |
| Tag | | | |
+-------+---------------+---------------+---------------------------+
| 98 | cose-sign | COSE_Sign | COSE Signed Data Object |
| | | | |
| 18 | cose-sign1 | COSE_Sign1 | COSE Single Signer Data |
| | | | Object |
| | | | |
| 96 | cose-encrypt | COSE_Encrypt | COSE Encrypted Data |
| | | | Object |
| | | | |
| 16 | cose-encrypt0 | COSE_Encrypt0 | COSE Single Recipient |
| | | | Encrypted Data Object |
| | | | |
| 97 | cose-mac | COSE_Mac | COSE Mac-ed Data Object |
| | | | |
| 17 | cose-mac0 | COSE_Mac0 | COSE Mac w/o Recipients |
| | | | Object |
+-------+---------------+---------------+---------------------------+
Table 1: COSE Message Identification
The following CDDL fragment identifies all of the top messages
defined in this document. Separate non-terminals are defined for the
tagged and the untagged versions of the messages.
COSE_Messages = COSE_Untagged_Message / COSE_Tagged_Message
COSE_Untagged_Message = COSE_Sign / COSE_Sign1 /
COSE_Encrypt / COSE_Encrypt0 /
COSE_Mac / COSE_Mac0
COSE_Tagged_Message = COSE_Sign_Tagged / COSE_Sign1_Tagged /
COSE_Encrypt_Tagged / COSE_Encrypt0_Tagged /
COSE_Mac_Tagged / COSE_Mac0_Tagged
3. Header Parameters
The structure of COSE has been designed to have two buckets of
information that are not considered to be part of the payload itself,
but are used for holding information about content, algorithms, keys,
or evaluation hints for the processing of the layer. These two
buckets are available for use in all of the structures except for
keys. While these buckets are present, they may not all be usable in
all instances. For example, while the protected bucket is defined as
part of the recipient structure, some of the algorithms used for
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recipient structures do not provide for authenticated data. If this
is the case, the protected bucket is left empty.
Both buckets are implemented as CBOR maps. The map key is a 'label'
(Section 1.4). The value portion is dependent on the definition for
the label. Both maps use the same set of label/value pairs. The
integer and string values for labels have been divided into several
sections with a standard range, a private range, and a range that is
dependent on the algorithm selected. The defined labels can be found
in the "COSE Header Parameters" IANA registry (Section 16.2).
Two buckets are provided for each layer:
protected: Contains parameters about the current layer that are to
be cryptographically protected. This bucket MUST be empty if it
is not going to be included in a cryptographic computation. This
bucket is encoded in the message as a binary object. This value
is obtained by CBOR encoding the protected map and wrapping it in
a bstr object. Senders SHOULD encode a zero length map as a zero
length string rather than as a zero length map (encoded as h'a0').
The zero length binary encoding is preferred because it is both
shorter and the version used in the serialization structures for
cryptographic computation. After encoding the map, the value is
wrapped in the binary object. Recipients MUST accept both a zero
length binary value and a zero length map encoded in the binary
value. The wrapping allows for the encoding of the protected map
to be transported with a greater chance that it will not be
altered in transit. (Badly behaved intermediates could decode and
re-encode, but this will result in a failure to verify unless the
re-encoded byte string is identical to the decoded byte string.)
This avoids the problem of all parties needing to be able to do a
common canonical encoding.
unprotected: Contains parameters about the current layer that are
not cryptographically protected.
Only parameters that deal with the current layer are to be placed at
that layer. As an example of this, the parameter 'content type'
describes the content of the message being carried in the message.
As such, this parameter is placed only in the content layer and is
not placed in the recipient or signature layers. In principle, one
should be able to process any given layer without reference to any
other layer. With the exception of the COSE_Sign structure, the only
data that needs to cross layers is the cryptographic key.
The buckets are present in all of the security objects defined in
this document. The fields in order are the 'protected' bucket (as a
CBOR 'bstr' type) and then the 'unprotected' bucket (as a CBOR 'map'
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type). The presence of both buckets is required. The parameters
that go into the buckets come from the IANA "COSE Header Parameters"
registry (Section 16.2). Some common parameters are defined in the
next section, but a number of parameters are defined throughout this
document.
Labels in each of the maps MUST be unique. When processing messages,
if a label appears multiple times, the message MUST be rejected as
malformed. Applications SHOULD verify that the same label does not
occur in both the protected and unprotected headers. If the message
is not rejected as malformed, attributes MUST be obtained from the
protected bucket before they are obtained from the unprotected
bucket.
The following CDDL fragment represents the two header buckets. A
group Headers is defined in CDDL that represents the two buckets in
which attributes are placed. This group is used to provide these two
fields consistently in all locations. A type is also defined which
represents the map of common headers.
Headers = (
protected : empty_or_serialized_map,
unprotected : header_map
)
header_map = {
Generic_Headers,
* label => values
}
empty_or_serialized_map = bstr .cbor header_map / bstr .size 0
3.1. Common COSE Headers Parameters
This section defines a set of common header parameters. A summary of
these parameters can be found in Table 2. This table should be
consulted to determine the value of label, and the type of the value.
The set of header parameters defined in this section are:
alg: This parameter is used to indicate the algorithm used for the
security processing. This parameter MUST be authenticated where
the ability to do so exists. This support is provided by AEAD
algorithms or construction (COSE_Sign, COSE_Sign0, COSE_Mac and
COSE_Mac0). This authentication can be done either by placing the
header in the protected header bucket or as part of the externally
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supplied data. The value is taken from the "COSE Algorithms"
Registry (see Section 16.4).
crit: The parameter is used to indicate which protected header
labels an application that is processing a message is required to
understand. Parameters defined in this document do not need to be
included as they should be understood by all implementations.
When present, this parameter MUST be placed in the protected
header bucket. The array MUST have at least one value in it.
Not all labels need to be included in the 'crit' parameter. The
rules for deciding which header labels are placed in the array
are:
* Integer labels in the range of 0 to 8 SHOULD be omitted.
* Integer labels in the range -1 to -128 can be omitted as they
are algorithm dependent. If an application can correctly
process an algorithm, it can be assumed that it will correctly
process all of the common parameters associated with that
algorithm. Integer labels in the range -129 to -65536 SHOULD
be included as these would be less common parameters that might
not be generally supported.
* Labels for parameters required for an application MAY be
omitted. Applications should have a statement if the label can
be omitted.
The header parameter values indicated by 'crit' can be processed
by either the security library code or by an application using a
security library; the only requirement is that the parameter is
processed. If the 'crit' value list includes a value for which
the parameter is not in the protected bucket, this is a fatal
error in processing the message.
content type: This parameter is used to indicate the content type of
the data in the payload or cipher text fields. Integers are from
the "CoAP Content-Formats" IANA registry table [COAP.Formats].
Text values following the syntax of "<type-name>/<subtype-name>"
where <type-name> and <subtype-name> are defined in Section 4.2 of
[RFC6838]. Leading and trailing whitespace is also omitted.
Textual content values along with parameters and subparameters can
be located using the IANA "Media Types" registry. Applications
SHOULD provide this parameter if the content structure is
potentially ambiguous.
kid: This parameter identifies one piece of data that can be used as
input to find the needed cryptographic key. The value of this
parameter can be matched against the 'kid' member in a COSE_Key
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structure. Other methods of key distribution can define an
equivalent field to be matched. Applications MUST NOT assume that
'kid' values are unique. There may be more than one key with the
same 'kid' value, so all of the keys associated with this 'kid'
may need to be checked. The internal structure of 'kid' values is
not defined and cannot be relied on by applications. Key
identifier values are hints about which key to use. This is not a
security critical field. For this reason, it can be placed in the
unprotected headers bucket.
IV: This parameter holds the Initialization Vector (IV) value. For
some symmetric encryption algorithms this may be referred to as a
nonce. The IV can be placed in the unprotected header as
modifying the IV will cause the decryption to yield plaintext that
is readily detectable as garbled.
Partial IV This parameter holds a part of the IV value. When using
the COSE_Encrypt0 structure, a portion of the IV can be part of
the context associated with the key. This field is used to carry
a value that causes the IV to be changed for each message. The IV
can be placed in the unprotected header as modifying the IV will
cause the decryption to yield plaintext that is readily detectable
as garbled. The 'Initialization Vector' and 'Partial
Initialization Vector' parameters MUST NOT both be present in the
same security layer.
The message IV is generated by the following steps:
1. Left pad the partial IV with zeros to the length of IV.
2. XOR the padded partial IV with the context IV.
counter signature: This parameter holds one or more counter
signature values. Counter signatures provide a method of having a
second party sign some data. The counter signature parameter can
occur as an unprotected attribute in any of the following
structures: COSE_Sign1, COSE_Signature, COSE_Encrypt,
COSE_recipient, COSE_Encrypt0, COSE_Mac and COSE_Mac0. These
structures all have the same beginning elements so that a
consistent calculation of the counter signature can be computed.
Details on computing counter signatures are found in Section 4.5.
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+-----------+-------+----------------+-------------+----------------+
| name | label | value type | value | description |
| | | | registry | |
+-----------+-------+----------------+-------------+----------------+
| alg | 1 | int / tstr | COSE | Cryptographic |
| | | | Algorithms | algorithm to |
| | | | registry | use |
| | | | | |
| crit | 2 | [+ label] | COSE Header | Critical |
| | | | Labels | headers to be |
| | | | registry | understood |
| | | | | |
| content | 3 | tstr / uint | CoAP | Content type |
| type | | | Content- | of the payload |
| | | | Formats or | |
| | | | Media Types | |
| | | | registry | |
| | | | | |
| kid | 4 | bstr | | Key identifier |
| | | | | |
| IV | 5 | bstr | | Full |
| | | | | Initialization |
| | | | | Vector |
| | | | | |
| Partial | 6 | bstr | | Partial |
| IV | | | | Initialization |
| | | | | Vector |
| | | | | |
| counter | 7 | COSE_Signature | | CBOR encoded |
| signature | | / [+ | | signature |
| | | COSE_Signature | | structure |
| | | ] | | |
+-----------+-------+----------------+-------------+----------------+
Table 2: Common Header Parameters
The CDDL fragment that represents the set of headers defined in this
section is given below. Each of the headers is tagged as optional
because they do not need to be in every map; headers required in
specific maps are discussed above.
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Generic_Headers = (
? 1 => int / tstr, ; algorithm identifier
? 2 => [+label], ; criticality
? 3 => tstr / int, ; content type
? 4 => bstr, ; key identifier
? 5 => bstr, ; IV
? 6 => bstr, ; Partial IV
? 7 => COSE_Signature / [+COSE_Signature] ; Counter signature
)
4. Signing Objects
COSE supports two different signature structures. COSE_Sign allows
for one or more signatures to be applied to the same content.
COSE_Sign1 is restricted to a single signer. The structures cannot
be converted between each other; as the signature computation
includes a parameter identifying which structure is being used, the
converted structure will fail signature validation.
4.1. Signing with One or More Signers
The COSE_Sign structure allows for one or more signatures to be
applied to a message payload. Parameters relating to the content and
parameters relating to the signature are carried along with the
signature itself. These parameters may be authenticated by the
signature, or just present. An example of a parameter about the
content is the content type. Examples of parameters about the
signature would be the algorithm and key used to create the signature
and counter signatures.
When more than one signature is present, the successful validation of
one signature associated with a given signer is usually treated as a
successful signature by that signer. However, there are some
application environments where other rules are needed. An
application that employs a rule other than one valid signature for
each signer must specify those rules. Also, where simple matching of
the signer identifier is not sufficient to determine whether the
signatures were generated by the same signer, the application
specification must describe how to determine which signatures were
generated by the same signer. Support for different communities of
recipients is the primary reason that signers choose to include more
than one signature. For example, the COSE_Sign structure might
include signatures generated with the Edwards Digital Signature
Algorithm (EdDSA) [I-D.irtf-cfrg-eddsa] signature algorithm and with
the Elliptic Curve Digital Signature Algorithm (ECDSA) [DSS]
signature algorithm. This allows recipients to verify the signature
associated with one algorithm or the other. (The original source of
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this text is [RFC5652].) More detailed information on multiple
signature evaluation can be found in [RFC5752].
The signature structure can be encoded either as tagged or untagged
depending on the context it will be used in. A tagged COSE_Sign
structure is identified by the CBOR tag TBD1. The CDDL fragment that
represents this is:
COSE_Sign_Tagged = #6.98(COSE_Sign)
A COSE Signed Message is defined in two parts. The CBOR object that
carries the body and information about the body is called the
COSE_Sign structure. The CBOR object that carries the signature and
information about the signature is called the COSE_Signature
structure. Examples of COSE Signed Messages can be found in
Appendix C.1.
The COSE_Sign structure is a CBOR array. The fields of the array in
order are:
protected as described in Section 3.
unprotected as described in Section 3.
payload contains the serialized content to be signed. If the
payload is not present in the message, the application is required
to supply the payload separately. The payload is wrapped in a
bstr to ensure that it is transported without changes. If the
payload is transported separately ("detached content"), then a nil
CBOR object is placed in this location and it is the
responsibility of the application to ensure that it will be
transported without changes.
Note: When a signature with message recovery algorithm is used
(Section 8), the maximum number of bytes that can be recovered is
the length of the payload. The size of the payload is reduced by
the number of bytes that will be recovered. If all of the bytes
of the payload are consumed, then the payload is encoded as a zero
length binary string rather than as being absent.
signatures is an array of signatures. Each signature is represented
as a COSE_Signature structure.
The CDDL fragment that represents the above text for COSE_Sign
follows.
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COSE_Sign = [
Headers,
payload : bstr / nil,
signatures : [+ COSE_Signature]
]
The COSE_Signature structure is a CBOR array. The fields of the
array in order are:
protected as described in Section 3.
unprotected as described in Section 3.
signature contains the computed signature value. The type of the
field is a bstr. Algorithms MUST specify padding if the signature
value is not a multiple of 8 bits.
The CDDL fragment that represents the above text for COSE_Signature
follows.
COSE_Signature = [
Headers,
signature : bstr
]
!
4.2. Signing with One Signer
The COSE_Sign1 signature structure is used when only one signature is
going to be placed on a message. The parameters dealing with the
content and the signature are placed in the same pair of buckets
rather than having the separation of COSE_Sign.
The structure can be encoded either tagged or untagged depending on
the context it will be used in. A tagged COSE_Sign1 structure is
identified by the CBOR tag TBD7. The CDDL fragment that represents
this is:
COSE_Sign1_Tagged = #6.18(COSE_Sign1)
The CBOR object that carries the body, the signature, and the
information about the body and signature is called the COSE_Sign1
structure. Examples of COSE_Sign1 messages can be found in
Appendix C.2.
The COSE_Sign1 structure is a CBOR array. The fields of the array in
order are:
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protected as described in Section 3.
unprotected as described in Section 3.
payload as described in Section 4.1.
signature contains the computed signature value. The type of the
field is a bstr.
The CDDL fragment that represents the above text for COSE_Sign1
follows.
COSE_Sign1 = [
Headers,
payload : bstr / nil,
signature : bstr
]
4.3. Externally Supplied Data
One of the features offered in the COSE document is the ability for
applications to provide additional data to be authenticated, but that
is not carried as part of the COSE object. The primary reason for
supporting this can be seen by looking at the CoAP message structure
[RFC7252], where the facility exists for options to be carried before
the payload. Examples of data that can be placed in this location
would be the CoAP code or CoAP options. If the data is in the header
section, then it is available for proxies to help in performing its
operations. For example, the Accept Option can be used by a proxy to
determine if an appropriate value is in the Proxy's cache. But the
sender can prevent a proxy from changing the set of values that it
will accept by including that value in the resulting authentication
tag. However, it may also be desired to protect these values so that
if they are modified in transit, it can be detected.
This document describes the process for using a byte array of
externally supplied authenticated data; however, the method of
constructing the byte array is a function of the application.
Applications that use this feature need to define how the externally
supplied authenticated data is to be constructed. Such a
construction needs to take into account the following issues:
o If multiple items are included, applications need to ensure that
the same byte string is not produced if there are different
inputs. This could occur by appending the strings 'AB' and 'CDE'
or by appending the strings 'ABC' and 'DE'. This is usually
addressed by making fields a fixed width and/or encoding the
length of the field as part of the output. Using options from
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CoAP [RFC7252] as an example, these fields use a TLV structure so
they can be concatenated without any problems.
o If multiple items are included, an order for the items needs to be
defined. Using options from CoAP as an example, an application
could state that the fields are to be ordered by the option
number.
o Applications need to ensure that the byte stream is going to be
the same on both sides. Using options from CoAP might give a
problem if the same relative numbering is kept. An intermediate
node could insert or remove an option, changing how the relative
number is done. An application would need to specify that the
relative number must be re-encoded to be relative only to the
options that are in the external data.
4.4. Signing and Verification Process
In order to create a signature, a well-defined byte stream is needed.
The Sig_struture is used to create the canonical form. This signing
and verification process takes in the body information (COSE_Sign or
COSE_Sign1), the signer information (COSE_Signature), and the
application data (external source). A Sig_structure is a CBOR array.
The fields of the Sig_struture in order are:
1. A text string identifying the context of the signature. The
context string is:
"Signature" for signatures using the COSE_Signature structure.
"Signature1" for signatures using the COSE_Sign1 structure.
"CounterSignature" for signatures used as counter signature
attributes.
2. The protected attributes from the body structure encoded in a
bstr type. If there are no protected attributes, a bstr of
length zero is used.
3. The protected attributes from the signer structure encoded in a
bstr type. If there are no protected attributes, a bstr of
length zero is used. This field is omitted for the COSE_Sign1
signature structure.
4. The protected attributes from the application encoded in a bstr
type. If this field is not supplied, it defaults to a zero
length binary string. (See Section 4.3 for application guidance
on constructing this field.)
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5. The payload to be signed encoded in a bstr type. The payload is
placed here independent of how it is transported.
The CDDL fragment that describes the above text is.
Sig_structure = [
context : "Signature" / "Signature1" / "CounterSignature",
body_protected : empty_or_serialized_map,
? sign_protected : empty_or_serialized_map,
external_aad : bstr,
payload : bstr
]
How to compute a signature:
1. Create a Sig_structure and populate it with the appropriate
fields.
2. Create the value ToBeSigned by encoding the Sig_structure to a
byte string, using the encoding described in Section 14.
3. Call the signature creation algorithm passing in K (the key to
sign with), alg (the algorithm to sign with), and ToBeSigned (the
value to sign).
4. Place the resulting signature value in the 'signature' field of
the array.
The steps for verifying a signature are:
1. Create a Sig_structure object and populate it with the
appropriate fields.
2. Create the value ToBeSigned by encoding the Sig_structure to a
byte string, using the encoding described in Section 14.
3. Call the signature verification algorithm passing in K (the key
to verify with), alg (the algorithm used sign with), ToBeSigned
(the value to sign), and sig (the signature to be verified).
In addition to performing the signature verification, the application
may also perform the appropriate checks to ensure that the key is
correctly paired with the signing identity and that the signing
identity is authorized before performing actions.
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4.5. Computing Counter Signatures
Counter signatures provide a method of associating different
signature generated by different signers with some piece of content.
This is normally used to provide a signature on a signature allowing
for a proof that a signature existed at a given time (i.e., a
Timestamp). In this document, we allow for counter signatures to
exist in a greater number of environments. As an example, it is
possible to place a counter signature in the unprotected attributes
of a COSE_Encrypt object. This would allow for an intermediary to
either verify that the encrypted byte stream has not been modified,
without being able to decrypt it, or for the intermediary to assert
that an encrypted byte stream either existed at a given time or
passed through it in terms of routing (i.e., a proxy signature).
An example of a counter signature on a signature can be found in
Appendix C.1.3. An example of a counter signature in an encryption
object can be found in Appendix C.3.3.
The creation and validation of counter signatures over the different
items relies on the fact that the structure of the objects have the
same structure. The elements are a set of protected attributes, a
set of unprotected attributes, and a body, in that order. This means
that the Sig_structure can be used in a uniform manner to get the
byte stream for processing a signature. If the counter signature is
going to be computed over a COSE_Encrypt structure, the
body_protected and payload items can be mapped into the Sig_structure
in the same manner as from the COSE_Sign structure.
It should be noted that only a signature algorithm with appendix (see
Section 8) can be used for counter signatures. This is because the
body should be able to be processed without having to evaluate the
counter signature, and this is not possible for signature schemes
with message recovery.
5. Encryption Objects
COSE supports two different encryption structures. COSE_Encrypt0 is
used when a recipient structure is not needed because the key to be
used is known implicitly. COSE_Encrypt is used the rest of the time.
This includes cases where there are multiple recipients or a
recipient algorithm other than direct is used.
5.1. Enveloped COSE Structure
The enveloped structure allows for one or more recipients of a
message. There are provisions for parameters about the content and
parameters about the recipient information to be carried in the
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message. The protected parameters associated with the content are
authenticated by the content encryption algorithm. The protected
parameters associated with the recipient are authenticated by the
recipient algorithm (when the algorithm supports it). Examples of
parameters about the content are the type of the content and the
content encryption algorithm. Examples of parameters about the
recipient are the recipient's key identifier and the recipient's
encryption algorithm.
The same techniques and structures are used for encrypting both the
plain text and the keys. This is different from the approach used by
both CMS [RFC5652] and JSON Web Encryption (JWE) [RFC7516] where
different structures are used for the content layer and for the
recipient layer. Two structures are defined: COSE_Encrypt to hold
the encrypted content and COSE_recipient to hold the encrypted keys
for recipients. Examples of encrypted messages can be found in
Appendix C.3.
The COSE_Encrypt structure can be encoded either tagged or untagged
depending on the context it will be used in. A tagged COSE_Encrypt
structure is identified by the CBOR tag TBD2. The CDDL fragment that
represents this is:
COSE_Encrypt_Tagged = #6.96(COSE_Encrypt)
The COSE_Encrypt structure is a CBOR array. The fields of the array
in order are:
protected as described in Section 3.
unprotected as described in Section 3. '
ciphertext contains the cipher text encoded as a bstr. If the
cipher text is to be transported independently of the control
information about the encryption process (i.e., detached content)
then the field is encoded as a nil value.
recipients contains an array of recipient information structures.
The type for the recipient information structure is a
COSE_recipient.
The CDDL fragment that corresponds to the above text is:
COSE_Encrypt = [
Headers,
ciphertext : bstr / nil,
recipients : [+COSE_recipient]
]
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The COSE_recipient structure is a CBOR array. The fields of the
array in order are:
protected as described in Section 3.
unprotected as described in Section 3.
ciphertext contains the encrypted key encoded as a bstr. All
encoded keys are symetric keys, the binary value of the key is the
content. If there is not an encrypted key, then this field is
encoded as a nil value.
recipients contains an array of recipient information structures.
The type for the recipient information structure is a
COSE_recipient. (An example of this can be found in Appendix B.)
If there are no recipient information structures, this element is
absent.
The CDDL fragment that corresponds to the above text for
COSE_recipient is:
COSE_recipient = [
Headers,
ciphertext : bstr / nil,
? recipients : [+COSE_recipient]
]
5.1.1. Content Key Distribution Methods
An encrypted message consists of an encrypted content and an
encrypted CEK for one or more recipients. The CEK is encrypted for
each recipient, using a key specific to that recipient. The details
of this encryption depend on which class the recipient algorithm
falls into. Specific details on each of the classes can be found in
Section 12. A short summary of the five content key distribution
methods is:
direct: The CEK is the same as the identified previously distributed
symmetric key or derived from a previously distributed secret. No
CEK is transported in the message.
symmetric key-encryption keys: The CEK is encrypted using a
previously distributed symmetric KEK.
key agreement: The recipient's public key and a sender's private key
are used to generate a pairwise secret, a KDF is applied to derive
a key, and then the CEK is either the derived key or encrypted by
the derived key.
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key transport: The CEK is encrypted with the recipient's public key.
No key transport algorithms are defined in this document.
passwords: The CEK is encrypted in a KEK that is derived from a
password. No password algorithms are defined in this document.
5.2. Single Recipient Encrypted
The COSE_Encrypt0 encrypted structure does not have the ability to
specify recipients of the message. The structure assumes that the
recipient of the object will already know the identity of the key to
be used in order to decrypt the message. If a key needs to be
identified to the recipient, the enveloped structure ought to be
used.
Examples of encrypted messages can be found in Appendix C.3.
The COSE_Encrypt0 structure can be encoded either tagged or untagged
depending on the context it will be used in. A tagged COSE_Encrypt0
structure is identified by the CBOR tag TBD3. The CDDL fragment that
represents this is:
COSE_Encrypt0_Tagged = #6.16(COSE_Encrypt0)
The COSE_Encrypt0 structure is a CBOR array. The fields of the array
in order are:
protected as described in Section 3.
unprotected as described in Section 3.
ciphertext as described in Section 5.1.
The CDDL fragment for COSE_Encrypt0 that corresponds to the above
text is:
COSE_Encrypt0 = [
Headers,
ciphertext : bstr / nil,
]
5.3. How to encrypt and decrypt for AEAD Algorithms
The encryption algorithm for AEAD algorithms is fairly simple. The
first step is to create a consistent byte stream for the
authenticated data structure. For this purpose, we use an
Enc_structure. The Enc_structure is a CBOR array. The fields of the
Enc_structure in order are:
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1. A text string identifying the context of the authenticated data
structure. The context string is:
"Encrypt0" for the content encryption of a COSE_Encrypt0 data
structure.
"Encrypt" for the first layer of a COSE_Encrypt data structure
(i.e., for content encryption).
"Enc_Recipient" for a recipient encoding to be placed in an
COSE_Encrypt data structure.
"Mac_Recipient" for a recipient encoding to be placed in a MACed
message structure.
"Rec_Recipient" for a recipient encoding to be placed in a
recipient structure.
2. The protected attributes from the body structure encoded in a
bstr type. If there are no protected attributes, a bstr of
length zero is used.
3. The protected attributes from the application encoded in a bstr
type. If this field is not supplied, it defaults to a zero
length bstr. (See Section 4.3 for application guidance on
constructing this field.)
The CDDL fragment that describes the above text is:
Enc_structure = [
context : "Encrypt" / "Encrypt0" / "Enc_Recipient" /
"Mac_Recipient" / "Rec_Recipient",
protected : empty_or_serialized_map,
external_aad : bstr
]
How to encrypt a message:
1. Create an Enc_structure and populate it with the appropriate
fields.
2. Encode the Enc_structure to a byte stream (AAD), using the
encoding described in Section 14.
3. Determine the encryption key (K). This step is dependent on the
class of recipient algorithm being used. For:
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No Recipients: The key to be used is determined by the algorithm
and key at the current layer. Examples are key transport keys
Section 12.3, key wrap keys Section 12.2.1 or pre-shared
secrets.
Direct Encryption and Direct Key Agreement: The key is
determined by the key and algorithm in the recipient
structure. The encryption algorithm and size of the key to be
used are inputs into the KDF used for the recipient. (For
direct, the KDF can be thought of as the identity operation.)
Examples of these algorithms are found in Section 12.1.2 and
Section 12.4.1.
Other: The key is randomly or pseudo-randomly generated.
4. Call the encryption algorithm with K (the encryption key), P (the
plain text) and AAD. Place the returned cipher text into the
'ciphertext' field of the structure.
5. For recipients of the message, recursively perform the encryption
algorithm for that recipient, using K (the encryption key) as the
plain text.
How to decrypt a message:
1. Create a Enc_structure and populate it with the appropriate
fields.
2. Encode the Enc_structure to a byte stream (AAD), using the
encoding described in Section 14.
3. Determine the decryption key. This step is dependent on the
class of recipient algorithm being used. For:
No Recipients: The key to be used is determined by the algorithm
and key at the current layer. Examples are key transport keys
Section 12.3, key wrap keys Section 12.2.1 or pre-shared
secrets.
Direct Encryption and Direct Key Agreement: The key is
determined by the key and algorithm in the recipient
structure. The encryption algorithm and size of the key to be
used are inputs into the KDF used for the recipient. (For
direct, the KDF can be thought of as the identity operation.)
Examples of these algorithms are found in Section 12.1.2 and
Section 12.4.1.
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Other: The key is determined by decoding and decrypting one of
the recipient structures.
4. Call the decryption algorithm with K (the decryption key to use),
C (the cipher text) and AAD.
5.4. How to encrypt and decrypt for AE Algorithms
How to encrypt a message:
1. Verify that the 'protected' field is empty.
2. Verify that there was no external additional authenticated data
supplied for this operation.
3. Determine the encryption key. This step is dependent on the
class of recipient algorithm being used. For:
No Recipients: The key to be used is determined by the algorithm
and key at the current layer. Examples are key transport keys
Section 12.3, key wrap keys Section 12.2.1 or pre-shared
secrets.
Direct Encryption and Direct Key Agreement: The key is
determined by the key and algorithm in the recipient
structure. The encryption algorithm and size of the key to be
used are inputs into the KDF used for the recipient. (For
direct, the KDF can be thought of as the identity operation.)
Examples of these algorithms are found in Section 12.1.2 and
Section 12.4.1.
Other: The key is randomly generated.
4. Call the encryption algorithm with K (the encryption key to use)
and the P (the plain text). Place the returned cipher text into
the 'ciphertext' field of the structure.
5. For recipients of the message, recursively perform the encryption
algorithm for that recipient, using K (the encryption key) as the
plain text.
How to decrypt a message:
1. Verify that the 'protected' field is empty.
2. Verify that there was no external additional authenticated data
supplied for this operation.
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3. Determine the decryption key. This step is dependent on the
class of recipient algorithm being used. For:
No Recipients: The key to be used is determined by the algorithm
and key at the current layer. Examples are key transport keys
Section 12.3, key wrap keys Section 12.2.1 or pre-shared
secrets.
Direct Encryption and Direct Key Agreement: The key is
determined by the key and algorithm in the recipient
structure. The encryption algorithm and size of the key to be
used are inputs into the KDF used for the recipient. (For
direct, the KDF can be thought of as the identity operation.)
Examples of these algorithms are found in Section 12.1.2 and
Section 12.4.1.
Other: The key is determined by decoding and decrypting one of
the recipient structures.
4. Call the decryption algorithm with K (the decryption key to use),
and C (the cipher text).
6. MAC Objects
COSE supports two different MAC structures. COSE_MAC0 is used when a
recipient structure is not needed because the key to be used is
implicitly known. COSE_MAC is used for all other cases. These
include a requirement for multiple recipients, the key being unknown,
and a recipient algorithm of other than direct.
In this section, we describe the structure and methods to be used
when doing MAC authentication in COSE. This document allows for the
use of all of the same classes of recipient algorithms as are allowed
for encryption.
When using MAC operations, there are two modes in which they can be
used. The first is just a check that the content has not been
changed since the MAC was computed. Any class of recipient algorithm
can be used for this purpose. The second mode is to both check that
the content has not been changed since the MAC was computed, and to
use the recipient algorithm to verify who sent it. The classes of
recipient algorithms that support this are those that use a pre-
shared secret or do static-static key agreement (without the key wrap
step). In both of these cases, the entity that created and sent the
message MAC can be validated. (This knowledge of sender assumes that
there are only two parties involved and you did not send the message
to yourself.) The origination property can be obtained with both of
the MAC message structures.
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6.1. MACed Message with Recipients
The multiple recipient MACed message uses two structures, the
COSE_Mac structure defined in this section for carrying the body and
the COSE_recipient structure (Section 5.1) to hold the key used for
the MAC computation. Examples of MACed messages can be found in
Appendix C.5.
The MAC structure can be encoded either tagged or untagged depending
on the context it will be used in. A tagged COSE_Mac structure is
identified by the CBOR tag TBD4. The CDDL fragment that represents
this is:
COSE_Mac_Tagged = #6.97(COSE_Mac)
The COSE_Mac structure is a CBOR array. The fields of the array in
order are:
protected as described in Section 3.
unprotected as described in Section 3.
payload contains the serialized content to be MACed. If the payload
is not present in the message, the application is required to
supply the payload separately. The payload is wrapped in a bstr
to ensure that it is transported without changes. If the payload
is transported separately (i.e., detached content), then a nil
CBOR value is placed in this location and it is the responsibility
of the application to ensure that it will be transported without
changes.
tag contains the MAC value.
recipients as described in Section 5.1.
The CDDL fragment that represents the above text for COSE_Mac
follows.
COSE_Mac = [
Headers,
payload : bstr / nil,
tag : bstr,
recipients :[+COSE_recipient]
]
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6.2. MACed Messages with Implicit Key
In this section, we describe the structure and methods to be used
when doing MAC authentication for those cases where the recipient is
implicitly known.
The MACed message uses the COSE_Mac0 structure defined in this
section for carrying the body. Examples of MACed messages with an
implicit key can be found in Appendix C.6.
The MAC structure can be encoded either tagged or untagged depending
on the context it will be used in. A tagged COSE_Mac0 structure is
identified by the CBOR tag TBD6. The CDDL fragment that represents
this is:
COSE_Mac0_Tagged = #6.17(COSE_Mac0)
The COSE_Mac0 structure is a CBOR array. The fields of the array in
order are:
protected as described in Section 3.
unprotected as described in Section 3.
payload as described in Section 6.1.
tag contains the MAC value.
The CDDL fragment that corresponds to the above text is:
COSE_Mac0 = [
Headers,
payload : bstr / nil,
tag : bstr,
]
6.3. How to compute and verify a MAC
In order to get a consistent encoding of the data to be
authenticated, the MAC_structure is used to have a canonical form.
The MAC_structure is a CBOR array. The fields of the MAC_structure
in order are:
1. A text string that identifies the structure that is being
encoded. This string is "MAC" for the COSE_Mac structure. This
string is "MAC0" for the COSE_Mac0 structure.
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2. The protected attributes from the COSE_MAC structure. If there
are no protected attributes, a zero length bstr is used.
3. The protected attributes from the application encoded as a bstr
type. If this field is not supplied, it defaults to a zero
length binary string. (See Section 4.3 for application guidance
on constructing this field.)
4. The payload to be MAC-ed encoded in a bstr type. The payload is
placed here independent of how it is transported.
The CDDL fragment that corresponds to the above text is:
MAC_structure = [
context : "MAC" / "MAC0",
protected : empty_or_serialized_map,
external_aad : bstr,
payload : bstr
]
The steps to compute a MAC are:
1. Create a MAC_structure and populate it with the appropriate
fields.
2. Create the value ToBeMaced by encoding the MAC_structure to a
byte stream, using the encoding described in Section 14.
3. Call the MAC creation algorithm passing in K (the key to use),
alg (the algorithm to MAC with) and ToBeMaced (the value to
compute the MAC on).
4. Place the resulting MAC in the 'tag' field of the COSE_Mac or
COSE_Mac0 structure.
5. Encrypt and encode the MAC key for each recipient of the message.
The steps to verify a MAC are:
1. Create a MAC_structure object and populate it with the
appropriate fields.
2. Create the value ToBeMaced by encoding the MAC_structure to a
byte stream, using the encoding described in Section 14.
3. Obtain the cryptographic key from one of the recipients of the
message.
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4. Call the MAC creation algorithm passing in K (the key to use),
alg (the algorithm to MAC with) and ToBeMaced (the value to
compute the MAC on).
5. Compare the MAC value to the 'tag' field of the COSE_Mac or
COSE_Mac0 structure.
7. Key Objects
A COSE Key structure is built on a CBOR map object. The set of
common parameters that can appear in a COSE Key can be found in the
IANA "COSE Key Common Parameters" registry (Section 16.5).
Additional parameters defined for specific key types can be found in
the IANA "COSE Key Type Parameters" registry (Section 16.6).
A COSE Key Set uses a CBOR array object as its underlying type. The
values of the array elements are COSE Keys. A Key Set MUST have at
least one element in the array. Examples of Key Sets can be found in
Appendix C.7.
Each element in a key set MUST be processed independently. If one
element in a key set is either malformed or uses a key that is not
understood by an application, that key is ignored and the other keys
are processed normally.
The element "kty" is a required element in a COSE_Key map.
The CDDL grammar describing COSE_Key and COSE_KeySet is:
COSE_Key = {
1 => tstr / int, ; kty
? 2 => bstr, ; kid
? 3 => tstr / int, ; alg
? 4 => [+ (tstr / int) ], ; key_ops
? 5 => bstr, ; Base IV
* label => values
}
COSE_KeySet = [+COSE_Key]
7.1. COSE Key Common Parameters
This document defines a set of common parameters for a COSE Key
object. Table 3 provides a summary of the parameters defined in this
section. There are also parameters that are defined for specific key
types. Key type specific parameters can be found in Section 13.
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+---------+-------+----------------+------------+-------------------+
| name | label | CBOR type | registry | description |
+---------+-------+----------------+------------+-------------------+
| kty | 1 | tstr / int | COSE Key | Identification of |
| | | | Common | the key type |
| | | | Parameters | |
| | | | | |
| alg | 3 | tstr / int | COSE | Key usage |
| | | | Algorithm | restriction to |
| | | | Values | this algorithm |
| | | | | |
| kid | 2 | bstr | | Key |
| | | | | Identification |
| | | | | value - match to |
| | | | | kid in message |
| | | | | |
| key_ops | 4 | [+ (tstr/int)] | | Restrict set of |
| | | | | permissible |
| | | | | operations |
| | | | | |
| Base IV | 5 | bstr | | Base IV to be |
| | | | | xor-ed with |
| | | | | Partial IVs |
+---------+-------+----------------+------------+-------------------+
Table 3: Key Map Labels
kty: This parameter is used to identify the family of keys for this
structure, and thus the set of key type specific parameters to be
found. The set of values defined in this document can be found in
Table 21. This parameter MUST be present in a key object.
Implementations MUST verify that the key type is appropriate for
the algorithm being processed. The key type MUST be included as
part of the trust decision process.
alg: This parameter is used to restrict the algorithm that is used
with the key. If this parameter is present in the key structure,
the application MUST verify that this algorithm matches the
algorithm for which the key is being used. If the algorithms do
not match, then this key object MUST NOT be used to perform the
cryptographic operation. Note that the same key can be in a
different key structure with a different or no algorithm
specified, however this is considered to be a poor security
practice.
kid: This parameter is used to give an identifier for a key. The
identifier is not structured and can be anything from a user
provided string to a value computed on the public portion of the
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key. This field is intended for matching against a 'kid'
parameter in a message in order to filter down the set of keys
that need to be checked.
key_ops: This parameter is defined to restrict the set of operations
that a key is to be used for. The value of the field is an array
of values from Table 4. Algorithms define the values of key ops
that are permitted to appear and are required for specific
operations. The set of values matches that in [RFC7517] and
[W3C.WebCrypto].
Base IV: This parameter is defined to carry the base portion of an
IV. It is designed to be used with the partial IV header
parameter defined in Section 3.1. This field provides the ability
to associate a partial IV with a key that is then modified on a
per message basis with the partial IV.
Extreme care needs to be taken when using a Base IV in an
application. Many encryption algorithms lose security if the same
IV is used twice.
If different keys are derived for each sender, using the same base
IV with partial IVs starting at zero is likely to ensure that the
IV would not be used twice for a single key. If different keys
are derived for each sender, starting at the same base IV is
likely to satisfy this condition. If the same key is used for
multiple senders, then the application needs to provide for a
method of dividing the IV space up between the senders. This
could be done by providing a different base point to start from or
a different partial IV to start with and restricting the number of
messages to be sent before re-keying.
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+---------+-------+-------------------------------------------------+
| name | value | description |
+---------+-------+-------------------------------------------------+
| sign | 1 | The key is used to create signatures. Requires |
| | | private key fields. |
| | | |
| verify | 2 | The key is used for verification of signatures. |
| | | |
| encrypt | 3 | The key is used for key transport encryption. |
| | | |
| decrypt | 4 | The key is used for key transport decryption. |
| | | Requires private key fields. |
| | | |
| wrap | 5 | The key is used for key wrapping. |
| key | | |
| | | |
| unwrap | 6 | The key is used for key unwrapping. Requires |
| key | | private key fields. |
| | | |
| derive | 7 | The key is used for deriving keys. Requires |
| key | | private key fields. |
| | | |
| derive | 8 | The key is used for deriving bits not to be |
| bits | | used as a key. Requires private key fields. |
| | | |
| MAC | 9 | The key is used for creating MACs. |
| create | | |
| | | |
| MAC | 10 | The key is used for validating MACs. |
| verify | | |
+---------+-------+-------------------------------------------------+
Table 4: Key Operation Values
8. Signature Algorithms
There are two signature algorithm schemes. The first is signature
with appendix. In this scheme, the message content is processed and
a signature is produced, the signature is called the appendix. This
is the scheme used by algorithms such as ECDSA and RSASSA-PSS. (In
fact the SSA in RSASSA-PSS stands for Signature Scheme with
Appendix.)
The signature functions for this scheme are:
signature = Sign(message content, key)
valid = Verification(message content, key, signature)
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The second scheme is signature with message recovery. (An example of
such an algorithm is [PVSig].) In this scheme, the message content
is processed, but part of it is included in the signature. Moving
bytes of the message content into the signature allows for smaller
signatures, the signature size is still potentially large, but the
message content has shrunk. This has implications for systems
implementing these algorithms and for applications that use them.
The first is that the message content is not fully available until
after a signature has been validated. Until that point the part of
the message contained inside of the signature is unrecoverable. The
second is that the security analysis of the strength of the signature
is very much based on the structure of the message content. Messages
that are highly predictable require additional randomness to be
supplied as part of the signature process. In the worst case, it
becomes the same as doing a signature with appendix. Finally, in the
event that multiple signatures are applied to a message, all of the
signature algorithms are going to be required to consume the same
number of bytes of message content. This means that mixing of the
different schemes in a single message is not supported, and if a
recovery signature scheme is used, then the same amount of content
needs to be consumed by all of the signatures.
The signature functions for this scheme are:
signature, message sent = Sign(message content, key)
valid, message content = Verification(message sent, key, signature)
Signature algorithms are used with the COSE_Signature and COSE_Sign1
structures. At this time, only signatures with appendixes are
defined for use with COSE, however considerable interest has been
expressed in using a signature with message recovery algorithm due to
the effective size reduction that is possible. Implementations will
need to keep this in mind for later possible integration.
8.1. ECDSA
ECDSA [DSS] defines a signature algorithm using ECC. Implementations
SHOULD use a deterministic version of ECDSA such as the one defined
in [RFC6979]. The use of a deterministic signature algorithms allows
for systems to avoid relying on random number generators in order to
avoid generating the same value of 'k' (the per-message random
value). Biased generation of the value be attacked and collisions
will lead to leaked keys. It additionally allows for doing
deterministic tests for the signature algorithm. The use of
deterministic ECDSA does not lessen the need to to have good random
number generation when creating the private key.
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The ECDSA signature algorithm is parameterized with a hash function
(h). In the event that the length of the hash function output is
greater than the group of the key, the left-most bytes of the hash
output are used.
The algorithms defined in this document can be found in Table 5.
+-------+-------+---------+------------------+
| name | value | hash | description |
+-------+-------+---------+------------------+
| ES256 | -7 | SHA-256 | ECDSA w/ SHA-256 |
| | | | |
| ES384 | -35 | SHA-384 | ECDSA w/ SHA-384 |
| | | | |
| ES512 | -36 | SHA-512 | ECDSA w/ SHA-512 |
+-------+-------+---------+------------------+
Table 5: ECDSA Algorithm Values
This document defines ECDSA to work only with the curves P-256, P-384
and P-521. This document requires that the curves be encoded using
the 'EC2' (2 coordinate Elliptic Curve) key type. Implementations
need to check that the key type and curve are correct when creating
and verifying a signature. Other documents can define it to work
with other curves and points in the future.
In order to promote interoperability, it is suggested that SHA-256 be
used only with curve P-256, SHA-384 be used only with curve P-384 and
SHA-512 be used with curve P-521. This is aligned with the
recommendation in Section 4 of [RFC5480].
The signature algorithm results in a pair of integers (R, S). These
integers will the same length as length of the key used for the
signature process. The signature is encoded by converting the
integers into byte strings of the same length as the key size. The
length is rounded up to the nearest byte and is left padded with zero
bits to get to the correct length. The two integers are then
concatenated together to form a byte string that is the resulting
signature.
Using the function defined in [I-D.moriarty-pkcs1] the signature is:
Signature = I2OSP(R, n) | I2OSP(S, n)
where n = ceiling(key_length / 8)
When using a COSE key for this algorithm, the following checks are
made:
o The 'kty' field MUST be present and it MUST be 'EC2'.
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o If the 'alg' field is present, it MUST match the ECDSA signature
algorithm being used.
o If the 'key_ops' field is present, it MUST include 'sign' when
creating an ECDSA signature.
o If the 'key_ops' field is present, it MUST include 'verify' when
verifying an ECDSA signature.
8.1.1. Security Considerations
The security strength of the signature is no greater than the minimum
of the security strength associated with the bit length of the key
and the security strength of the hash function.
Note: Use of this technique is a good idea even when good random
number generation exists. Doing so both reduces the possibility of
having the same value of 'k' in two signature operations and allows
for reproducible signature values, which helps testing.
There are two substitution attacks that can theoretically be mounted
against the ECDSA signature algorithm.
o Changing the curve used to validate the signature: If one changes
the curve used to validate the signature, then potentially one
could have a two messages with the same signature each computed
under a different curve. The only requirement on the new curve is
that its order be the same as the old one and it be acceptable to
the client. An example would be to change from using the curve
secp256r1 (aka P-256) to using secp256k1. (Both are 256 bit
curves.) We current do not have any way to deal with this version
of the attack except to restrict the overall set of curves that
can be used.
o Change the hash function used to validate the signature: If one
has either two different hash functions of the same length, or one
can truncate a hash function down, then one could potentially find
collisions between the hash functions rather than within a single
hash function. (For example, truncating SHA-512 to 256 bits might
collide with a SHA-256 bit hash value.) As the hash algorithm is
part of the signature algorithm identifier, this attack is
mitigated by including signature algorithm identifier in the
protected header.
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8.2. Edwards-curve Digital Signature Algorithms (EdDSA)
[I-D.irtf-cfrg-eddsa] describes the elliptic curve signature scheme
Edwards-curve Digital Signature Algorithm (EdDSA). In that document,
the signature algorithm is instantiated using parameters for
edwards25519 and edwards448 curves. The document additionally
describes two variants of the EdDSA algorithm: Pure EdDSA, where no
hash function is applied to the content before signing and, HashEdDSA
where a hash function is applied to the content before signing and
the result of that hash function is signed. For the EdDSA, the
content to be signed (either the message or the pre-hash value) is
processed twice inside of the signature algorithm. For use with
COSE, only the pure EdDSA version is used. This is because it is not
expected that extremely large contents are going to be needed and,
based on the arrangement of the message structure, the entire message
is going to need to be held in memory in order to create or verify a
signature. This means that there does not appear to be a need to be
able to do block updates of the hash, followed by eliminating the
message from memory. Applications can provide the same features by
defining the content of the message as a hash value and transporting
the COSE object (with the hash value) and the content as separate
items.
The algorithms defined in this document can be found in Table 6. A
single signature algorithm is defined, which can be used for multiple
curves.
+-------+-------+-------------+
| name | value | description |
+-------+-------+-------------+
| EdDSA | -8 | EdDSA |
+-------+-------+-------------+
Table 6: EdDSA Algorithm Values
[I-D.irtf-cfrg-eddsa] describes the method of encoding the signature
value.
When using a COSE key for this algorithm the following checks are
made:
o The 'kty' field MUST be present and it MUST be 'OKP' (Octet Key
Pair).
o The 'crv' field MUST be present, and it MUST be a curve defined
for this signature algorithm.
o If the 'alg' field is present, it MUST match 'EdDSA'.
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o If the 'key_ops' field is present, it MUST include 'sign' when
creating an EdDSA signature.
o If the 'key_ops' field is present, it MUST include 'verify' when
verifying an EdDSA signature.
8.2.1. Security Considerations
How public values are computed is not the same when looking at EdDSA
and ECDH, for this reason they should not be used with the other
algorithm.
If batch signature verification is performed, a well-seeded
cryptographic random number generator is REQUIRED. Signing and non-
batch signature verification are deterministic operations and do not
need random numbers of any kind.
9. Message Authentication (MAC) Algorithms
Message Authentication Codes (MACs) provide data authentication and
integrity protection. They provide either no or very limited data
origination. A MAC, for example, be used to prove the identity of
the sender to a third party.
MACs use the same scheme as signature with appendix algorithms. The
message content is processed and an authentication code is produced.
The authentication code is frequently called a tag.
The MAC functions are:
tag = MAC_Create(message content, key)
valid = MAC_Verify(message content, key, tag)
MAC algorithms can be based on either a block cipher algorithm (i.e.,
AES-MAC) or a hash algorithm (i.e., HMAC). This document defines a
MAC algorithm using each of these constructions.
MAC algorithms are used in the COSE_Mac and COSE_Mac0 structures.
9.1. Hash-based Message Authentication Codes (HMAC)
The Hash-based Message Authentication Code algorithm (HMAC)
[RFC2104][RFC4231] was designed to deal with length extension
attacks. The algorithm was also designed to allow for new hash
algorithms to be directly plugged in without changes to the hash
function. The HMAC design process has been shown as solid since,
while the security of hash algorithms such as MD5 has decreased over
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time, the security of HMAC combined with MD5 has not yet been shown
to be compromised [RFC6151].
The HMAC algorithm is parameterized by an inner and outer padding, a
hash function (h), and an authentication tag value length. For this
specification, the inner and outer padding are fixed to the values
set in [RFC2104]. The length of the authentication tag corresponds
to the difficulty of producing a forgery. For use in constrained
environments, we define a set of HMAC algorithms that are truncated.
There are currently no known issues with truncation, however the
security strength of the message tag is correspondingly reduced in
strength. When truncating, the left-most tag length bits are kept
and transmitted.
The algorithms defined in this document can be found in Table 7.
+-----------+-------+---------+----------+--------------------------+
| name | value | Hash | Tag | description |
| | | | Length | |
+-----------+-------+---------+----------+--------------------------+
| HMAC | 4 | SHA-256 | 64 | HMAC w/ SHA-256 |
| 256/64 | | | | truncated to 64 bits |
| | | | | |
| HMAC | 5 | SHA-256 | 256 | HMAC w/ SHA-256 |
| 256/256 | | | | |
| | | | | |
| HMAC | 6 | SHA-384 | 384 | HMAC w/ SHA-384 |
| 384/384 | | | | |
| | | | | |
| HMAC | 7 | SHA-512 | 512 | HMAC w/ SHA-512 |
| 512/512 | | | | |
+-----------+-------+---------+----------+--------------------------+
Table 7: HMAC Algorithm Values
Some recipient algorithms carry the key while others derive a key
from secret data. For those algorithms that carry the key (such as
AES-KeyWrap), the size of the HMAC key SHOULD be the same size as the
underlying hash function. For those algorithms that derive the key
(such as ECDH), the derived key MUST be the same size as the
underlying hash function.
When using a COSE key for this algorithm, the following checks are
made:
o The 'kty' field MUST be present and it MUST be 'Symmetric'.
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o If the 'alg' field is present, it MUST match the HMAC algorithm
being used.
o If the 'key_ops' field is present, it MUST include 'MAC create'
when creating an HMAC authentication tag.
o If the 'key_ops' field is present, it MUST include 'MAC verify'
when verifying an HMAC authentication tag.
Implementations creating and validating MAC values MUST validate that
the key type, key length, and algorithm are correct and appropriate
for the entities involved.
9.1.1. Security Considerations
HMAC has proved to be resistant to attack even when used with
weakened hash algorithms. The current best known attack appears is
to brute force the key. This means that key size is going to be
directly related to the security of an HMAC operation.
9.2. AES Message Authentication Code (AES-CBC-MAC)
AES-CBC-MAC is defined in [MAC]. (Note this is not the same
algorithm as AES-CMAC [RFC4493]).
AES-CBC-MAC is parameterized by the key length, the authentication
tag length and the IV used. For all of these algorithms, the IV is
fixed to all zeros. We provide an array of algorithms for various
key lengths and tag lengths. The algorithms defined in this document
are found in Table 8.
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+-------------+-------+----------+----------+-----------------------+
| name | value | key | tag | description |
| | | length | length | |
+-------------+-------+----------+----------+-----------------------+
| AES-MAC | 14 | 128 | 64 | AES-MAC 128 bit key, |
| 128/64 | | | | 64-bit tag |
| | | | | |
| AES-MAC | 15 | 256 | 64 | AES-MAC 256 bit key, |
| 256/64 | | | | 64-bit tag |
| | | | | |
| AES-MAC | 25 | 128 | 128 | AES-MAC 128 bit key, |
| 128/128 | | | | 128-bit tag |
| | | | | |
| AES-MAC | 26 | 256 | 128 | AES-MAC 256 bit key, |
| 256/128 | | | | 128-bit tag |
+-------------+-------+----------+----------+-----------------------+
Table 8: AES-MAC Algorithm Values
Keys may be obtained either from a key structure or from a recipient
structure. Implementations creating and validating MAC values MUST
validate that the key type, key length and algorithm are correct and
appropriate for the entities involved.
When using a COSE key for this algorithm, the following checks are
made:
o The 'kty' field MUST be present and it MUST be 'Symmetric'.
o If the 'alg' field is present, it MUST match the AES-MAC algorithm
being used.
o If the 'key_ops' field is present, it MUST include 'MAC create'
when creating an AES-MAC authentication tag.
o If the 'key_ops' field is present, it MUST include 'MAC verify'
when verifying an AES-MAC authentication tag.
9.2.1. Security Considerations
A number of attacks exist against CBC-MAC that need to be considered.
-
o A single key must only be used for messages of a fixed and known
length. If this is not the case, an attacker will be able to
generate a message with a valid tag given two message and tag
pairs. This can be addressed by using different keys for
different length messages. The current structure mitigates this
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problem, as a specific encoding structure that includes lengths is
built and signed. (CMAC also addresses this issue.)
o When using CBC mode, if the same key is used for both encryption
and authentication operations, an attacker can produce messages
with a valid authentication code.
o If the IV can be modified, then messages can be forged. This is
addressed by fixing the IV to all zeros.
10. Content Encryption Algorithms
Content Encryption Algorithms provide data confidentiality for
potentially large blocks of data using a symmetric key. They provide
integrity on the data that was encrypted, however they provide either
no or very limited data origination. (One cannot, for example, be
used to prove the identity of the sender to a third party.) The
ability to provide data origination is linked to how the CEK is
obtained.
COSE restricts the set of legal content encryption algorithms to
those that support authentication both of the content and additional
data. The encryption process will generate some type of
authentication value, but that value may be either explicit or
implicit in terms of the algorithm definition. For simplicity sake,
the authentication code will normally be defined as being appended to
the cipher text stream. The encryption functions are:
ciphertext = Encrypt(message content, key, additional data)
valid, message content = Decrypt(cipher text, key, additional data)
Most AEAD algorithms are logically defined as returning the message
content only if the decryption is valid. Many but not all
implementations will follow this convention. The message content
MUST NOT be used if the decryption does not validate.
These algorithms are used in COSE_Encrypt and COSE_Encrypt0.
10.1. AES GCM
The GCM mode is a generic authenticated encryption block cipher mode
defined in [AES-GCM]. The GCM mode is combined with the AES block
encryption algorithm to define an AEAD cipher.
The GCM mode is parameterized by the size of the authentication tag
and the size of the nonce. This document fixes the size of the nonce
at 96 bits. The size of the authentication tag is limited to a small
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set of values. For this document however, the size of the
authentication tag is fixed at 128 bits.
The set of algorithms defined in this document are in Table 9.
+---------+-------+------------------------------------------+
| name | value | description |
+---------+-------+------------------------------------------+
| A128GCM | 1 | AES-GCM mode w/ 128-bit key, 128-bit tag |
| | | |
| A192GCM | 2 | AES-GCM mode w/ 192-bit key, 128-bit tag |
| | | |
| A256GCM | 3 | AES-GCM mode w/ 256-bit key, 128-bit tag |
+---------+-------+------------------------------------------+
Table 9: Algorithm Value for AES-GCM
Keys may be obtained either from a key structure or from a recipient
structure. Implementations encrypting and decrypting MUST validate
that the key type, key length and algorithm are correct and
appropriate for the entities involved.
When using a COSE key for this algorithm, the following checks are
made:
o The 'kty' field MUST be present and it MUST be 'Symmetric'.
o If the 'alg' field is present, it MUST match the AES-GCM algorithm
being used.
o If the 'key_ops' field is present, it MUST include 'encrypt' or
'wrap key' when encrypting.
o If the 'key_ops' field is present, it MUST include 'decrypt' or
'unwrap key' when decrypting.
10.1.1. Security Considerations
When using AES-GCM, the following restrictions MUST be enforced:
o The key and nonce pair MUST be unique for every message encrypted.
o The total amount of data encrypted for a single key MUST NOT
exceed 2^39 - 256 bits. An explicit check is required only in
environments where it is expected that it might be exceeded.
Consideration was given to supporting smaller tag values; the
constrained community would desire tag sizes in the 64-bit range.
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Doing so drastically changes both the maximum messages size
(generally not an issue) and the number of times that a key can be
used. Given that CCM is the usual mode for constrained environments,
restricted modes are not supported.
10.2. AES CCM
Counter with CBC-MAC (CCM) is a generic authentication encryption
block cipher mode defined in [RFC3610]. The CCM mode is combined
with the AES block encryption algorithm to define a commonly used
content encryption algorithm used in constrained devices.
The CCM mode has two parameter choices. The first choice is M, the
size of the authentication field. The choice of the value for M
involves a trade-off between message growth (from the tag) and the
probably that an attacker can undetectably modify a message. The
second choice is L, the size of the length field. This value
requires a trade-off between the maximum message size and the size of
the Nonce.
It is unfortunate that the specification for CCM specified L and M as
a count of bytes rather than a count of bits. This leads to possible
misunderstandings where AES-CCM-8 is frequently used to refer to a
version of CCM mode where the size of the authentication is 64 bits
and not 8 bits. These values have traditionally been specified as
bit counts rather than byte counts. This document will follow the
convention of using bit counts so that it is easier to compare the
different algorithms presented in this document.
We define a matrix of algorithms in this document over the values of
L and M. Constrained devices are usually operating in situations
where they use short messages and want to avoid doing recipient
specific cryptographic operations. This favors smaller values of
both L and M. Less constrained devices will want to be able to use
larger messages and are more willing to generate new keys for every
operation. This favors larger values of L and M.
The following values are used for L:
16 bits (2) limits messages to 2^16 bytes (64 KiB) in length. This
is sufficiently long for messages in the constrained world. The
nonce length is 13 bytes allowing for 2^(13*8) possible values of
the nonce without repeating.
64 bits (8) limits messages to 2^64 bytes in length. The nonce
length is 7 bytes allowing for 2^56 possible values of the nonce
without repeating.
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The following values are used for M:
64 bits (8) produces a 64-bit authentication tag. This implies that
there is a 1 in 2^64 chance that a modified message will
authenticate.
128 bits (16) produces a 128-bit authentication tag. This implies
that there is a 1 in 2^128 chance that a modified message will
authenticate.
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+--------------------+-------+----+-----+-----+---------------------+
| name | value | L | M | k | description |
+--------------------+-------+----+-----+-----+---------------------+
| AES-CCM-16-64-128 | 10 | 16 | 64 | 128 | AES-CCM mode |
| | | | | | 128-bit key, 64-bit |
| | | | | | tag, 13-byte nonce |
| | | | | | |
| AES-CCM-16-64-256 | 11 | 16 | 64 | 256 | AES-CCM mode |
| | | | | | 256-bit key, 64-bit |
| | | | | | tag, 13-byte nonce |
| | | | | | |
| AES-CCM-64-64-128 | 12 | 64 | 64 | 128 | AES-CCM mode |
| | | | | | 128-bit key, 64-bit |
| | | | | | tag, 7-byte nonce |
| | | | | | |
| AES-CCM-64-64-256 | 13 | 64 | 64 | 256 | AES-CCM mode |
| | | | | | 256-bit key, 64-bit |
| | | | | | tag, 7-byte nonce |
| | | | | | |
| AES-CCM-16-128-128 | 30 | 16 | 128 | 128 | AES-CCM mode |
| | | | | | 128-bit key, |
| | | | | | 128-bit tag, |
| | | | | | 13-byte nonce |
| | | | | | |
| AES-CCM-16-128-256 | 31 | 16 | 128 | 256 | AES-CCM mode |
| | | | | | 256-bit key, |
| | | | | | 128-bit tag, |
| | | | | | 13-byte nonce |
| | | | | | |
| AES-CCM-64-128-128 | 32 | 64 | 128 | 128 | AES-CCM mode |
| | | | | | 128-bit key, |
| | | | | | 128-bit tag, 7-byte |
| | | | | | nonce |
| | | | | | |
| AES-CCM-64-128-256 | 33 | 64 | 128 | 256 | AES-CCM mode |
| | | | | | 256-bit key, |
| | | | | | 128-bit tag, 7-byte |
| | | | | | nonce |
+--------------------+-------+----+-----+-----+---------------------+
Table 10: Algorithm Values for AES-CCM
Keys may be obtained either from a key structure or from a recipient
structure. Implementations encrypting and decrypting MUST validate
that the key type, key length and algorithm are correct and
appropriate for the entities involved.
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When using a COSE key for this algorithm, the following checks are
made:
o The 'kty' field MUST be present and it MUST be 'Symmetric'.
o If the 'alg' field is present, it MUST match the AES-CCM algorithm
being used.
o If the 'key_ops' field is present, it MUST include 'encrypt' or
'wrap key' when encrypting.
o If the 'key_ops' field is present, it MUST include 'decrypt' or
'unwrap key' when decrypting.
10.2.1. Security Considerations
When using AES-CCM, the following restrictions MUST be enforced:
o The key and nonce pair MUST be unique for every message encrypted.
Note that the value of L influences the number of unique nonces.
o The total number of times the AES block cipher is used MUST NOT
exceed 2^61 operations. This limitation is the sum of times the
block cipher is used in computing the MAC value and in performing
stream encryption operations. An explicit check is required only
in environments where it is expected that it might be exceeded.
[RFC3610] additionally calls out one other consideration of note. It
is possible to do a pre-computation attack against the algorithm in
cases where portions of the plaintext are highly predictable. This
reduces the security of the key size by half. Ways to deal with this
attack include adding a random portion to the nonce value and/or
increasing the key size used. Using a portion of the nonce for a
random value will decrease the number of messages that a single key
can be used for. Increasing the key size may require more resources
in the constrained device. See sections 5 and 10 of [RFC3610] for
more information.
10.3. ChaCha20 and Poly1305
ChaCha20 and Poly1305 combined together is an AEAD mode that is
defined in [RFC7539]. This is an algorithm defined to be a cipher
that is not AES and thus would not suffer from any future weaknesses
found in AES. These cryptographic functions are designed to be fast
in software-only implementations.
The ChaCha20/Poly1305 AEAD construction defined in [RFC7539] has no
parameterization. It takes a 256-bit key and a 96-bit nonce, as well
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as the plain text and additional data as inputs and produces the
cipher text as an option. We define one algorithm identifier for
this algorithm in Table 11.
+-------------------+-------+---------------------------------------+
| name | value | description |
+-------------------+-------+---------------------------------------+
| ChaCha20/Poly1305 | 24 | ChaCha20/Poly1305 w/ 256-bit key, |
| | | 128-bit tag |
+-------------------+-------+---------------------------------------+
Table 11: Algorithm Value for AES-GCM
Keys may be obtained either from a key structure or from a recipient
structure. Implementations encrypting and decrypting MUST validate
that the key type, key length and algorithm are correct and
appropriate for the entities involved.
When using a COSE key for this algorithm, the following checks are
made:
o The 'kty' field MUST be present and it MUST be 'Symmetric'.
o If the 'alg' field is present, it MUST match the ChaCha20/Poly1305
algorithm being used.
o If the 'key_ops' field is present, it MUST include 'encrypt' or
'wrap key' when encrypting.
o If the 'key_ops' field is present, it MUST include 'decrypt' or
'unwrap key' when decrypting.
10.3.1. Security Considerations
The pair of key, nonce MUST be unique for every invocation of the
algorithm. Nonce counters are considered to be an acceptable way of
ensuring that they are unique.
11. Key Derivation Functions (KDF)
Key Derivation Functions (KDFs) are used to take some secret value
and generate a different one. The secret value comes in three
flavors:
o Secrets that are uniformly random: This is the type of secret that
is created by a good random number generator.
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o Secrets that are not uniformly random: This is type of secret that
is created by operations like key agreement.
o Secrets that are not random: This is the type of secret that
people generate for things like passwords.
General KDF functions work well with the first type of secret, can do
reasonably well with the second type of secret, and generally do
poorly with the last type of secret. None of the KDF functions in
this section are designed to deal with the type of secrets that are
used for passwords. Functions like PBES2 [I-D.moriarty-pkcs5-v2dot1]
need to be used for that type of secret.
The same KDF function can be setup to deal with the first two types
of secrets in a different way. The KDF function defined in
Section 11.1 is such a function. This is reflected in the set of
algorithms defined for HKDF.
When using KDF functions, one component that is included is context
information. Context information is used to allow for different
keying information to be derived from the same secret. The use of
context based keying material is considered to be a good security
practice.
This document defines a single context structure and a single KDF
function. These elements are used for all of the recipient
algorithms defined in this document that require a KDF process.
These algorithms are defined in Section 12.1.2, Section 12.4.1, and
Section 12.5.1.
11.1. HMAC-based Extract-and-Expand Key Derivation Function (HKDF)
The HKDF key derivation algorithm is defined in [RFC5869].
The HKDF algorithm takes these inputs:
secret - a shared value that is secret. Secrets may be either
previously shared or derived from operations like a DH key
agreement.
salt - an optional value that is used to change the generation
process. The salt value can be either public or private. If the
salt is public and carried in the message, then the 'salt'
algorithm header parameter defined in Table 13 is used. While
[RFC5869] suggests that the length of the salt be the same as the
length of the underlying hash value, any amount of salt will
improve the security as different key values will be generated.
This parameter is protected by being included in the key
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computation and does not need to be separately authenticated. The
salt value does not need to be unique for every message sent.
length - the number of bytes of output that need to be generated.
context information - Information that describes the context in
which the resulting value will be used. Making this information
specific to the context in which the material is going to be used
ensures that the resulting material will always be tied to that
usage. The context structure defined in Section 11.2 is used by
the KDF functions in this document.
PRF - The underlying pseudo-random function to be used in the HKDF
algorithm. The PRF is encoded into the HKDF algorithm selection.
HKDF is defined to use HMAC as the underlying PRF. However, it is
possible to use other functions in the same construct to provide a
different KDF function that is more appropriate in the constrained
world. Specifically, one can use AES-CBC-MAC as the PRF for the
expand step, but not for the extract step. When using a good random
shared secret of the correct length, the extract step can be skipped.
For the AES algorithm versions, the extract step is always skipped.
The extract step cannot be skipped if the secret is not uniformly
random, for example, if it is the result of an ECDH key agreement
step. (This implies that the AES HKDF version cannot be used with
ECDH.) If the extract step is skipped, the 'salt' value is not used
as part of the HKDF functionality.
The algorithms defined in this document are found in Table 12.
+---------------+-----------------+---------------------------------+
| name | PRF | description |
+---------------+-----------------+---------------------------------+
| HKDF SHA-256 | HMAC with | HKDF using HMAC SHA-256 as the |
| | SHA-256 | PRF |
| | | |
| HKDF SHA-512 | HMAC with | HKDF using HMAC SHA-512 as the |
| | SHA-512 | PRF |
| | | |
| HKDF AES- | AES-CBC-MAC-128 | HKDF using AES-MAC as the PRF |
| MAC-128 | | w/ 128-bit key |
| | | |
| HKDF AES- | AES-CBC-MAC-256 | HKDF using AES-MAC as the PRF |
| MAC-256 | | w/ 256-bit key |
+---------------+-----------------+---------------------------------+
Table 12: HKDF algorithms
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+------+-------+------+-------------------------------+-------------+
| name | label | type | algorithm | description |
+------+-------+------+-------------------------------+-------------+
| salt | -20 | bstr | direct+HKDF-SHA-256, direct | Random salt |
| | | | +HKDF-SHA-512, direct+HKDF- | |
| | | | AES-128, direct+HKDF-AES-256, | |
| | | | ECDH-ES+HKDF-256, ECDH- | |
| | | | ES+HKDF-512, ECDH- | |
| | | | SS+HKDF-256, ECDH- | |
| | | | SS+HKDF-512, ECDH-ES+A128KW, | |
| | | | ECDH-ES+A192KW, ECDH- | |
| | | | ES+A256KW, ECDH-SS+A128KW, | |
| | | | ECDH-SS+A192KW, ECDH- | |
| | | | SS+A256KW | |
+------+-------+------+-------------------------------+-------------+
Table 13: HKDF Algorithm Parameters
11.2. Context Information Structure
The context information structure is used to ensure that the derived
keying material is "bound" to the context of the transaction. The
context information structure used here is based on that defined in
[SP800-56A]. By using CBOR for the encoding of the context
information structure, we automatically get the same type and length
separation of fields that is obtained by the use of ASN.1. This
means that there is no need to encode the lengths for the base
elements as it is done by the encoding used in JOSE (Section 4.6.2 of
[RFC7518]).
The context information structure refers to PartyU and PartyV as the
two parties that are doing the key derivation. Unless the
application protocol defines differently, we assign PartyU to the
entity that is creating the message and PartyV to the entity that is
receiving the message. By doing this association, different keys
will be derived for each direction as the context information is
different in each direction.
The context structure is built from information that is known to both
entities. This information can be obtained from a variety of
sources:
o Fields can be defined by the application. This is commonly used
to assign fixed names to parties, but can be used for other items
such as nonces.
o Fields can be defined by usage of the output. Examples of this
are the algorithm and key size that are being generated.
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o Fields can be defined by parameters from the message. We define a
set of parameters in Table 14 that can be used to carry the values
associated with the context structure. Examples of this are
identities and nonce values. These parameters are designed to be
placed in the unprotected bucket of the recipient structure.
(They do not need to be in the protected bucket since they already
are included in the cryptographic computation by virtue of being
included in the context structure.)
+----------+-------+------+---------------------------+-------------+
| name | label | type | algorithm | description |
+----------+-------+------+---------------------------+-------------+
| PartyU | -21 | bstr | direct+HKDF-SHA-256, | Party U |
| identity | | | direct+HKDF-SHA-512, | identity |
| | | | direct+HKDF-AES-128, | Information |
| | | | direct+HKDF-AES-256, | |
| | | | ECDH-ES+HKDF-256, ECDH- | |
| | | | ES+HKDF-512, ECDH- | |
| | | | SS+HKDF-256, ECDH- | |
| | | | SS+HKDF-512, ECDH- | |
| | | | ES+A128KW, ECDH- | |
| | | | ES+A192KW, ECDH- | |
| | | | ES+A256KW, ECDH- | |
| | | | SS+A128KW, ECDH- | |
| | | | SS+A192KW, ECDH-SS+A256KW | |
| | | | | |
| PartyU | -22 | bstr | direct+HKDF-SHA-256, | Party U |
| nonce | | / | direct+HKDF-SHA-512, | provided |
| | | int | direct+HKDF-AES-128, | nonce |
| | | | direct+HKDF-AES-256, | |
| | | | ECDH-ES+HKDF-256, ECDH- | |
| | | | ES+HKDF-512, ECDH- | |
| | | | SS+HKDF-256, ECDH- | |
| | | | SS+HKDF-512, ECDH- | |
| | | | ES+A128KW, ECDH- | |
| | | | ES+A192KW, ECDH- | |
| | | | ES+A256KW, ECDH- | |
| | | | SS+A128KW, ECDH- | |
| | | | SS+A192KW, ECDH-SS+A256KW | |
| | | | | |
| PartyU | -23 | bstr | direct+HKDF-SHA-256, | Party U |
| other | | | direct+HKDF-SHA-512, | other |
| | | | direct+HKDF-AES-128, | provided |
| | | | direct+HKDF-AES-256, | information |
| | | | ECDH-ES+HKDF-256, ECDH- | |
| | | | ES+HKDF-512, ECDH- | |
| | | | SS+HKDF-256, ECDH- | |
| | | | SS+HKDF-512, ECDH- | |
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| | | | ES+A128KW, ECDH- | |
| | | | ES+A192KW, ECDH- | |
| | | | ES+A256KW, ECDH- | |
| | | | SS+A128KW, ECDH- | |
| | | | SS+A192KW, ECDH-SS+A256KW | |
| | | | | |
| PartyV | -24 | bstr | direct+HKDF-SHA-256, | Party V |
| identity | | | direct+HKDF-SHA-512, | identity |
| | | | direct+HKDF-AES-128, | Information |
| | | | direct+HKDF-AES-256, | |
| | | | ECDH-ES+HKDF-256, ECDH- | |
| | | | ES+HKDF-512, ECDH- | |
| | | | SS+HKDF-256, ECDH- | |
| | | | SS+HKDF-512, ECDH- | |
| | | | ES+A128KW, ECDH- | |
| | | | ES+A192KW, ECDH- | |
| | | | ES+A256KW, ECDH- | |
| | | | SS+A128KW, ECDH- | |
| | | | SS+A192KW, ECDH-SS+A256KW | |
| | | | | |
| PartyV | -25 | bstr | direct+HKDF-SHA-256, | Party V |
| nonce | | / | direct+HKDF-SHA-512, | provided |
| | | int | direct+HKDF-AES-128, | nonce |
| | | | direct+HKDF-AES-256, | |
| | | | ECDH-ES+HKDF-256, ECDH- | |
| | | | ES+HKDF-512, ECDH- | |
| | | | SS+HKDF-256, ECDH- | |
| | | | SS+HKDF-512, ECDH- | |
| | | | ES+A128KW, ECDH- | |
| | | | ES+A192KW, ECDH- | |
| | | | ES+A256KW, ECDH- | |
| | | | SS+A128KW, ECDH- | |
| | | | SS+A192KW, ECDH-SS+A256KW | |
| | | | | |
| PartyV | -26 | bstr | direct+HKDF-SHA-256, | Party V |
| other | | | direct+HKDF-SHA-512, | other |
| | | | direct+HKDF-AES-128, | provided |
| | | | direct+HKDF-AES-256, | information |
| | | | ECDH-ES+HKDF-256, ECDH- | |
| | | | ES+HKDF-512, ECDH- | |
| | | | SS+HKDF-256, ECDH- | |
| | | | SS+HKDF-512, ECDH- | |
| | | | ES+A128KW, ECDH- | |
| | | | ES+A192KW, ECDH- | |
| | | | ES+A256KW, ECDH- | |
| | | | SS+A128KW, ECDH- | |
| | | | SS+A192KW, ECDH-SS+A256KW | |
+----------+-------+------+---------------------------+-------------+
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Table 14: Context Algorithm Parameters
We define a CBOR object to hold the context information. This object
is referred to as COSE_KDF_Context. The object is based on a CBOR
array type. The fields in the array are:
AlgorithmID This field indicates the algorithm for which the key
material will be used. This normally is either a Key Wrap
algorithm identifier or a Content Encryption algorithm identifier.
The values are from the "COSE Algorithm Value" registry. This
field is required to be present. The field exists in the context
information so that if the same environment is used for different
algorithms, then completely different keys will be generated for
each of those algorithms. (This practice means if algorithm A is
broken and thus is easier to find, the key derived for algorithm B
will not be the same as the key derived for algorithm A.)
PartyUInfo This field holds information about party U. The
PartyUInfo is encoded as a CBOR array. The elements of PartyUInfo
are encoded in the order presented, however if the element does
not exist no element is placed in the array. The elements of the
PartyUInfo array are:
identity This contains the identity information for party U. The
identities can be assigned in one of two manners. Firstly, a
protocol can assign identities based on roles. For example,
the roles of "client" and "server" may be assigned to different
entities in the protocol. Each entity would then use the
correct label for the data they send or receive. The second
way for a protocol to assign identities is to use a name based
on a naming system (i.e., DNS, X.509 names).
We define an algorithm parameter 'PartyU identity' that can be
used to carry identity information in the message. However,
identity information is often known as part of the protocol and
can thus be inferred rather than made explicit. If identity
information is carried in the message, applications SHOULD have
a way of validating the supplied identity information. The
identity information does not need to be specified and is set
to nil in that case.
nonce This contains a nonce value. The nonce can either be
implicit from the protocol or carried as a value in the
unprotected headers.
We define an algorithm parameter 'PartyU nonce' that can be
used to carry this value in the message However, the nonce
value could be determined by the application and the value
determined from elsewhere.
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This option does not need to be specified and is set to nil in
that case
other This contains other information that is defined by the
protocol.
This option does not need to be specified and is set to nil in
that case
PartyVInfo This field holds information about party V. The content
of the structure are the same as for the PartyUInfo but for party
V.
SuppPubInfo This field contains public information that is mutually
known to both parties.
keyDataLength This is set to the number of bits of the desired
output value. (This practice means if algorithm A can use two
different key lengths, the key derived for longer key size will
not contain the key for shorter key size as a prefix.)
protected This field contains the protected parameter field. If
there are no elements in the protected field, then use a zero
length bstr.
other This field is for free form data defined by the
application. An example is that an application could define
two different strings to be placed here to generate different
keys for a data stream vs a control stream. This field is
optional and will only be present if the application defines a
structure for this information. Applications that define this
SHOULD use CBOR to encode the data so that types and lengths
are correctly included.
SuppPrivInfo This field contains private information that is
mutually known private information. An example of this
information would be a pre-existing shared secret. (This could,
for example, be used in combination with an ECDH key agreement to
provide a secondary proof of identity.) The field is optional and
will only be present if the application defines a structure for
this information. Applications that define this SHOULD use CBOR
to encode the data so that types and lengths are correctly
included.
The following CDDL fragment corresponds to the text above.
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PartyInfo = (
identity : bstr / nil,
nonce : bstr / int / nil,
other : bstr / nil,
)
COSE_KDF_Context = [
AlgorithmID : int / tstr,
PartyUInfo : [ PartyInfo ],
PartyVInfo : [ PartyInfo ],
SuppPubInfo : [
keyDataLength : uint,
protected : empty_or_serialized_map,
? other : bstr
],
? SuppPrivInfo : bstr
]
12. Content Key Distribution Methods
Content key distribution methods (recipient algorithms) can be
defined into a number of different classes. COSE has the ability to
support many classes of recipient algorithms. In this section, a
number of classes are listed and then a set of algorithms are
specified for each of the classes. The names of the recipient
algorithm classes used here are the same as are defined in [RFC7516].
Other specifications use different terms for the recipient algorithm
classes or do not support some of the recipient algorithm classes.
12.1. Direct Encryption
The direct encryption class algorithms share a secret between the
sender and the recipient that is used either directly or after
manipulation as the CEK. When direct encryption mode is used, it
MUST be the only mode used on the message.
The COSE_Encrypt structure for the recipient is organized as follows:
o The 'protected' field MUST be a zero length item unless it is used
in the computation of the content key.
o The 'alg' parameter MUST be present.
o A parameter identifying the shared secret SHOULD be present.
o The 'ciphertext' field MUST be a zero length item.
o The 'recipients' field MUST be absent.
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12.1.1. Direct Key
This recipient algorithm is the simplest; the identified key is
directly used as the key for the next layer down in the message.
There are no algorithm parameters defined for this algorithm. The
algorithm identifier value is assigned in Table 15.
When this algorithm is used, the protected field MUST be zero length.
The key type MUST be 'Symmetric'.
+--------+-------+-------------------+
| name | value | description |
+--------+-------+-------------------+
| direct | -6 | Direct use of CEK |
+--------+-------+-------------------+
Table 15: Direct Key
12.1.1.1. Security Considerations
This recipient algorithm has several potential problems that need to
be considered:
o These keys need to have some method to be regularly updated over
time. All of the content encryption algorithms specified in this
document have limits on how many times a key can be used without
significant loss of security.
o These keys need to be dedicated to a single algorithm. There have
been a number of attacks developed over time when a single key is
used for multiple different algorithms. One example of this is
the use of a single key both for CBC encryption mode and CBC-MAC
authentication mode.
o Breaking one message means all messages are broken. If an
adversary succeeds in determining the key for a single message,
then the key for all messages is also determined.
12.1.2. Direct Key with KDF
These recipient algorithms take a common shared secret between the
two parties and applies the HKDF function (Section 11.1), using the
context structure defined in Section 11.2 to transform the shared
secret into the CEK. The 'protected' field can be of non-zero
length. Either the 'salt' parameter of HKDF or the partyU 'nonce'
parameter of the context structure MUST be present. The salt/nonce
parameter can be generated either randomly or deterministically. The
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requirement is that it be a unique value for the shared secret in
question.
If the salt/nonce value is generated randomly, then it is suggested
that the length of the random value be the same length as the hash
function underlying HKDF. While there is no way to guarantee that it
will be unique, there is a high probability that it will be unique.
If the salt/nonce value is generated deterministically, it can be
guaranteed to be unique and thus there is no length requirement.
A new IV must be used for each message if the same key is used. The
IV can be modified in a predictable manner, a random manner or an
unpredictable manner (i.e., encrypting a counter).
The IV used for a key can also be generated from the same HKDF
functionality as the key is generated. If HKDF is used for
generating the IV, the algorithm identifier is set to "IV-
GENERATION".
When these algorithms are used, the key type MUST be 'symmetric'.
The set of algorithms defined in this document can be found in
Table 16.
+---------------------+-------+-------------+-----------------------+
| name | value | KDF | description |
+---------------------+-------+-------------+-----------------------+
| direct+HKDF-SHA-256 | -10 | HKDF | Shared secret w/ HKDF |
| | | SHA-256 | and SHA-256 |
| | | | |
| direct+HKDF-SHA-512 | -11 | HKDF | Shared secret w/ HKDF |
| | | SHA-512 | and SHA-512 |
| | | | |
| direct+HKDF-AES-128 | -12 | HKDF AES- | Shared secret w/ AES- |
| | | MAC-128 | MAC 128-bit key |
| | | | |
| direct+HKDF-AES-256 | -13 | HKDF AES- | Shared secret w/ AES- |
| | | MAC-256 | MAC 256-bit key |
+---------------------+-------+-------------+-----------------------+
Table 16: Direct Key with KDF
When using a COSE key for this algorithm, the following checks are
made:
o The 'kty' field MUST be present and it MUST be 'Symmetric'.
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o If the 'alg' field is present, it MUST match the algorithm being
used.
o If the 'key_ops' field is present, it MUST include 'deriveKey' or
'deriveBits'.
12.1.2.1. Security Considerations
The shared secret needs to have some method to be regularly updated
over time. The shared secret forms the basis of trust. Although not
used directly, it should still be subject to scheduled rotation.
While these methods do not provide for perfect forward secrecy, as
the same shared secret is used for all of the keys generated, if the
key for any single message is discovered only the message (or series
of messages) using that derived key are compromised. A new key
derivation step will generate a new key which requires the same
amount of work to get the key.
12.2. Key Wrapping
In key wrapping mode, the CEK is randomly generated and that key is
then encrypted by a shared secret between the sender and the
recipient. All of the currently defined key wrapping algorithms for
COSE are AE algorithms. Key wrapping mode is considered to be
superior to direct encryption if the system has any capability for
doing random key generation. This is because the shared key is used
to wrap random data rather than data that has some degree of
organization and may in fact be repeating the same content. The use
of Key Wrapping loses the weak data origination that is provided by
the direct encryption algorithms.
The COSE_Encrypt structure for the recipient is organized as follows:
o The 'protected' field MUST be absent if the key wrap algorithm is
an AE algorithm.
o The 'recipients' field is normally absent, but can be used.
Applications MUST deal with a recipient field being present, not
being able to decrypt that recipient is an acceptable way of
dealing with it. Failing to process the message is not an
acceptable way of dealing with it.
o The plain text to be encrypted is the key from next layer down
(usually the content layer).
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o At a minimum, the 'unprotected' field MUST contain the 'alg'
parameter and SHOULD contain a parameter identifying the shared
secret.
12.2.1. AES Key Wrapping
The AES Key Wrapping algorithm is defined in [RFC3394]. This
algorithm uses an AES key to wrap a value that is a multiple of 64
bits. As such, it can be used to wrap a key for any of the content
encryption algorithms defined in this document. The algorithm
requires a single fixed parameter, the initial value. This is fixed
to the value specified in Section 2.2.3.1 of [RFC3394]. There are no
public parameters that vary on a per invocation basis. The protected
header field MUST be empty.
Keys may be obtained either from a key structure or from a recipient
structure. Implementations encrypting and decrypting MUST validate
that the key type, key length and algorithm are correct and
appropriate for the entities involved.
When using a COSE key for this algorithm, the following checks are
made:
o The 'kty' field MUST be present and it MUST be 'Symmetric'.
o If the 'alg' field is present, it MUST match the AES Key Wrap
algorithm being used.
o If the 'key_ops' field is present, it MUST include 'encrypt' or
'wrap key' when encrypting.
o If the 'key_ops' field is present, it MUST include 'decrypt' or
'unwrap key' when decrypting.
+--------+-------+----------+-----------------------------+
| name | value | key size | description |
+--------+-------+----------+-----------------------------+
| A128KW | -3 | 128 | AES Key Wrap w/ 128-bit key |
| | | | |
| A192KW | -4 | 192 | AES Key Wrap w/ 192-bit key |
| | | | |
| A256KW | -5 | 256 | AES Key Wrap w/ 256-bit key |
+--------+-------+----------+-----------------------------+
Table 17: AES Key Wrap Algorithm Values
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12.2.1.1. Security Considerations for AES-KW
The shared secret needs to have some method to be regularly updated
over time. The shared secret is the basis of trust.
12.3. Key Transport
Key transport mode is also called key encryption mode in some
standards. Key transport mode differs from key wrap mode in that it
uses an asymmetric encryption algorithm rather than a symmetric
encryption algorithm to protect the key. This document does not
define any key transport mode algorithms.
When using a key transport algorithm, the COSE_Encrypt structure for
the recipient is organized as follows:
o The 'protected' field MUST be absent.
o The plain text to be encrypted is the key from next layer down
(usually the content layer).
o At a minimum, the 'unprotected' field MUST contain the 'alg'
parameter and SHOULD contain a parameter identifying the
asymmetric key.
12.4. Direct Key Agreement
The 'direct key agreement' class of recipient algorithms uses a key
agreement method to create a shared secret. A KDF is then applied to
the shared secret to derive a key to be used in protecting the data.
This key is normally used as a CEK or MAC key, but could be used for
other purposes if more than two layers are in use (see Appendix B).
The most commonly used key agreement algorithm is Diffie-Hellman, but
other variants exist. Since COSE is designed for a store and forward
environment rather than an on-line environment, many of the DH
variants cannot be used as the receiver of the message cannot provide
any dynamic key material. One side-effect of this is that perfect
forward secrecy (see [RFC4949]) is not achievable. A static key will
always be used for the receiver of the COSE object.
Two variants of DH that are supported are:
Ephemeral-Static DH: where the sender of the message creates a
one-time DH key and uses a static key for the recipient. The use
of the ephemeral sender key means that no additional random input
is needed as this is randomly generated for each message.
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Static-Static DH: where a static key is used for both the sender
and the recipient. The use of static keys allows for recipient to
get a weak version of data origination for the message. When
static-static key agreement is used, then some piece of unique
data for the KDF is required to ensure that a different key is
created for each message.
When direct key agreement mode is used, there MUST be only one
recipient in the message. This method creates the key directly and
that makes it difficult to mix with additional recipients. If
multiple recipients are needed, then the version with key wrap needs
to be used.
The COSE_Encrypt structure for the recipient is organized as follows:
o At a minimum, headers MUST contain the 'alg' parameter and SHOULD
contain a parameter identifying the recipient's asymmetric key.
o The headers SHOULD identify the sender's key for the static-static
versions and MUST contain the sender's ephemeral key for the
ephemeral-static versions.
12.4.1. ECDH
The mathematics for Elliptic Curve Diffie-Hellman can be found in
[RFC6090]. In this document, the algorithm is extended to be used
with the two curves defined in [RFC7748].
ECDH is parameterized by the following:
o Curve Type/Curve: The curve selected controls not only the size of
the shared secret, but the mathematics for computing the shared
secret. The curve selected also controls how a point in the curve
is represented and what happens for the identity points on the
curve. In this specification, we allow for a number of different
curves to be used. A set of curves are defined in Table 22.
The math used to obtain the computed secret is based on the curve
selected and not on the ECDH algorithm. For this reason, a new
algorithm does not need to be defined for each of the curves.
o Computed Secret to Shared Secret: Once the computed secret is
known, the resulting value needs to be converted to a byte string
to run the KDF function. The X coordinate is used for all of the
curves defined in this document. For curves X25519 and X448, the
resulting value is used directly as it is a byte string of a known
length. For the P-256, P-384 and P-521 curves, the X coordinate
is run through the I2OSP function defined in [I-D.moriarty-pkcs1],
using the same computation for n as is defined in Section 8.1.
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o Ephemeral-static or static-static: The key agreement process may
be done using either a static or an ephemeral key for the sender's
side. When using ephemeral keys, the sender MUST generate a new
ephemeral key for every key agreement operation. The ephemeral
key is placed in the 'ephemeral key' parameter and MUST be present
for all algorithm identifiers that use ephemeral keys. When using
static keys, the sender MUST either generate a new random value or
otherwise create a unique value. For the KDF functions used, this
means either in the 'salt' parameter for HKDF (Table 13) or in the
'PartyU nonce' parameter for the context structure (Table 14) MUST
be present. (Both may be present if desired.) The value in the
parameter MUST be unique for the pair of keys being used. It is
acceptable to use a global counter that is incremented for every
static-static operation and use the resulting value. When using
static keys, the static key should be identified to the recipient.
The static key can be identified either by providing the key
('static key') or by providing a key identifier for the static key
('static key id'). Both of these parameters are defined in
Table 19.
o Key derivation algorithm: The result of an ECDH key agreement
process does not provide a uniformly random secret. As such, it
needs to be run through a KDF in order to produce a usable key.
Processing the secret through a KDF also allows for the
introduction of context material: how the key is going to be used,
and one-time material for static-static key agreement. All of the
algorithms defined in this document use one of the HKDF algorithms
defined in Section 11.1 with the context structure defined in
Section 11.2.
o Key Wrap algorithm: No key wrap algorithm is used. This is
represented in Table 18 as 'none'. The key size for the context
structure is the content layer encryption algorithm size.
The set of direct ECDH algorithms defined in this document are found
in Table 18.
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+-----------+-------+---------+------------+--------+---------------+
| name | value | KDF | Ephemeral- | Key | description |
| | | | Static | Wrap | |
+-----------+-------+---------+------------+--------+---------------+
| ECDH-ES + | -25 | HKDF - | yes | none | ECDH ES w/ |
| HKDF-256 | | SHA-256 | | | HKDF - |
| | | | | | generate key |
| | | | | | directly |
| | | | | | |
| ECDH-ES + | -26 | HKDF - | yes | none | ECDH ES w/ |
| HKDF-512 | | SHA-512 | | | HKDF - |
| | | | | | generate key |
| | | | | | directly |
| | | | | | |
| ECDH-SS + | -27 | HKDF - | no | none | ECDH SS w/ |
| HKDF-256 | | SHA-256 | | | HKDF - |
| | | | | | generate key |
| | | | | | directly |
| | | | | | |
| ECDH-SS + | -28 | HKDF - | no | none | ECDH SS w/ |
| HKDF-512 | | SHA-512 | | | HKDF - |
| | | | | | generate key |
| | | | | | directly |
+-----------+-------+---------+------------+--------+---------------+
Table 18: ECDH Algorithm Values
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+-----------+-------+----------+---------------------+--------------+
| name | label | type | algorithm | description |
+-----------+-------+----------+---------------------+--------------+
| ephemeral | -1 | COSE_Key | ECDH-ES+HKDF-256, | Ephemeral |
| key | | | ECDH-ES+HKDF-512, | Public key |
| | | | ECDH-ES+A128KW, | for the |
| | | | ECDH-ES+A192KW, | sender |
| | | | ECDH-ES+A256KW | |
| | | | | |
| static | -2 | COSE_Key | ECDH-SS+HKDF-256, | Static |
| key | | | ECDH-SS+HKDF-512, | Public key |
| | | | ECDH-SS+A128KW, | for the |
| | | | ECDH-SS+A192KW, | sender |
| | | | ECDH-SS+A256KW | |
| | | | | |
| static | -3 | bstr | ECDH-SS+HKDF-256, | Static |
| key id | | | ECDH-SS+HKDF-512, | Public key |
| | | | ECDH-SS+A128KW, | identifier |
| | | | ECDH-SS+A192KW, | for the |
| | | | ECDH-SS+A256KW | sender |
+-----------+-------+----------+---------------------+--------------+
Table 19: ECDH Algorithm Parameters
This document defines these algorithms to be used with the curves
P-256, P-384, P-521, X25519, and X448. Implementations MUST verify
that the key type and curve are correct. Different curves are
restricted to different key types. Implementations MUST verify that
the curve and algorithm are appropriate for the entities involved.
When using a COSE key for this algorithm, the following checks are
made:
o The 'kty' field MUST be present and it MUST be 'EC2' or 'OKP'.
o If the 'alg' field is present, it MUST match the Key Agreement
algorithm being used.
o If the 'key_ops' field is present, it MUST include 'derive key' or
'derive bits' for the private key.
o If the 'key_ops' field is present, it MUST be empty for the public
key.
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12.4.2. Security Considerations
Some method of checking that points provided from external entities
are valid. For the 'EC2' key format, this can be done by checking
that the x and y values form a point on the curve. For the 'OKP'
format, there is no simple way to do point validation.
Consideration was given to requiring that the public keys of both
entities be provided as part of the key derivation process. (As
recommended in section 6.1 of [RFC7748].) This was not done as COSE
is used in a store and forward format rather than in on line key
exchange. In order for this to be a problem, either the receiver
public key has to be chosen maliciously or the sender has to be
malicious. In either case, all security evaporates anyway.
A proof of possession of the private key associated with the public
key is recommended when a key is moved from untrusted to trusted.
(Either by the end user or by the entity that is responsible for
making trust statements on keys.)
12.5. Key Agreement with Key Wrap
Key Agreement with Key Wrapping uses a randomly generated CEK. The
CEK is then encrypted using a Key Wrapping algorithm and a key
derived from the shared secret computed by the key agreement
algorithm. The function for this would be:
encryptedKey = KeyWrap(KDF(DH-Shared, context), CEK)
The COSE_Encrypt structure for the recipient is organized as follows:
o The 'protected' field is fed into the KDF context structure.
o The plain text to be encrypted is the key from next layer down
(usually the content layer).
o The 'alg' parameter MUST be present in the layer.
o A parameter identifying the recipient's key SHOULD be present. A
parameter identifying the sender's key SHOULD be present.
12.5.1. ECDH
These algorithms are defined in Table 20.
ECDH with Key Agreement is parameterized by the same parameters as
for ECDH Section 12.4.1 with the following modifications:
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o Key Wrap Algorithm: Any of the key wrap algorithms defined in
Section 12.2.1 are supported. The size of the key used for the
key wrap algorithm is fed into the KDF function. The set of
identifiers are found in Table 20.
+-----------+-------+---------+------------+--------+---------------+
| name | value | KDF | Ephemeral- | Key | description |
| | | | Static | Wrap | |
+-----------+-------+---------+------------+--------+---------------+
| ECDH-ES + | -29 | HKDF - | yes | A128KW | ECDH ES w/ |
| A128KW | | SHA-256 | | | Concat KDF |
| | | | | | and AES Key |
| | | | | | wrap w/ 128 |
| | | | | | bit key |
| | | | | | |
| ECDH-ES + | -30 | HKDF - | yes | A192KW | ECDH ES w/ |
| A192KW | | SHA-256 | | | Concat KDF |
| | | | | | and AES Key |
| | | | | | wrap w/ 192 |
| | | | | | bit key |
| | | | | | |
| ECDH-ES + | -31 | HKDF - | yes | A256KW | ECDH ES w/ |
| A256KW | | SHA-256 | | | Concat KDF |
| | | | | | and AES Key |
| | | | | | wrap w/ 256 |
| | | | | | bit key |
| | | | | | |
| ECDH-SS + | -32 | HKDF - | no | A128KW | ECDH SS w/ |
| A128KW | | SHA-256 | | | Concat KDF |
| | | | | | and AES Key |
| | | | | | wrap w/ 128 |
| | | | | | bit key |
| | | | | | |
| ECDH-SS + | -33 | HKDF - | no | A192KW | ECDH SS w/ |
| A192KW | | SHA-256 | | | Concat KDF |
| | | | | | and AES Key |
| | | | | | wrap w/ 192 |
| | | | | | bit key |
| | | | | | |
| ECDH-SS + | -34 | HKDF - | no | A256KW | ECDH SS w/ |
| A256KW | | SHA-256 | | | Concat KDF |
| | | | | | and AES Key |
| | | | | | wrap w/ 256 |
| | | | | | bit key |
+-----------+-------+---------+------------+--------+---------------+
Table 20: ECDH Algorithm Values with Key Wrap
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When using a COSE key for this algorithm, the following checks are
made:
o The 'kty' field MUST be present and it MUST be 'EC2' or 'OKP'.
o If the 'alg' field is present, it MUST match the Key Agreement
algorithm being used.
o If the 'key_ops' field is present, it MUST include 'derive key' or
'derive bits' for the private key.
o If the 'key_ops' field is present, it MUST be empty for the public
key.
13. Key Object Parameters
The COSE_Key object defines a way to hold a single key object. It is
still required that the members of individual key types be defined.
This section of the document is where we define an initial set of
members for specific key types.
For each of the key types, we define both public and private members.
The public members are what is transmitted to others for their usage.
Private members allow for the archival of keys by individuals.
However, there are some circumstances in which private keys may be
distributed to entities in a protocol. Examples include: entities
that have poor random number generation, centralized key creation for
multi-cast type operations, and protocols in which a shared secret is
used as a bearer token for authorization purposes.
Key types are identified by the 'kty' member of the COSE_Key object.
In this document, we define four values for the member:
+-----------+-------+--------------------------------------------+
| name | value | description |
+-----------+-------+--------------------------------------------+
| OKP | 1 | Octet Key Pair |
| | | |
| EC2 | 2 | Elliptic Curve Keys w/ X,Y Coordinate pair |
| | | |
| Symmetric | 4 | Symmetric Keys |
| | | |
| Reserved | 0 | This value is reserved |
+-----------+-------+--------------------------------------------+
Table 21: Key Type Values
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13.1. Elliptic Curve Keys
Two different key structures could be defined for Elliptic Curve
keys. One version uses both an x and a y coordinate, potentially
with point compression ('EC2'). This is the traditional EC point
representation that is used in [RFC5480]. The other version uses
only the x coordinate as the y coordinate is either to be recomputed
or not needed for the key agreement operation ('OKP').
Applications MUST check that the curve and the key type are
consistent and reject a key if they are not.
+---------+----------+-------+------------------------------------+
| name | key type | value | description |
+---------+----------+-------+------------------------------------+
| P-256 | EC2 | 1 | NIST P-256 also known as secp256r1 |
| | | | |
| P-384 | EC2 | 2 | NIST P-384 also known as secp384r1 |
| | | | |
| P-521 | EC2 | 3 | NIST P-521 also known as secp521r1 |
| | | | |
| X25519 | OKP | 4 | X25519 for use w/ ECDH only |
| | | | |
| X448 | OKP | 5 | X448 for use w/ ECDH only |
| | | | |
| Ed25519 | OKP | 6 | Ed25519 for use w/ EdDSA only |
| | | | |
| Ed448 | OKP | 7 | Ed448 for use w/ EdDSA only |
+---------+----------+-------+------------------------------------+
Table 22: EC Curves
13.1.1. Double Coordinate Curves
The traditional way of sending EC curves has been to send either both
the x and y coordinates, or the x coordinate and a sign bit for the y
coordinate. The latter encoding has not been recommended in the IETF
due to potential IPR issues. However, for operations in constrained
environments, the ability to shrink a message by not sending the y
coordinate is potentially useful.
For EC keys with both coordinates, the 'kty' member is set to 2
(EC2). The key parameters defined in this section are summarized in
Table 23. The members that are defined for this key type are:
crv contains an identifier of the curve to be used with the key.
The curves defined in this document for this key type can be found
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in Table 22. Other curves may be registered in the future and
private curves can be used as well.
x contains the x coordinate for the EC point. The integer is
converted to an octet string as defined in [SEC1]. Leading zero
octets MUST be preserved.
y contains either the sign bit or the value of y coordinate for the
EC point. When encoding the value y, the integer is converted to
an octet string (as defined in [SEC1]) and encoded as a CBOR bstr.
Leading zero octets MUST be preserved. The compressed point
encoding is also supported. Compute the sign bit as laid out in
the Elliptic-Curve-Point-to-Octet-String Conversion function of
[SEC1]. If the sign bit is zero, then encode y as a CBOR false
value, otherwise encode y as a CBOR true value. The encoding of
the infinity point is not supported.
d contains the private key.
For public keys, it is REQUIRED that 'crv', 'x' and 'y' be present in
the structure. For private keys, it is REQUIRED that 'crv' and 'd'
be present in the structure. For private keys, it is RECOMMENDED
that 'x' and 'y' also be present, but they can be recomputed from the
required elements and omitting them saves on space.
+------+-------+-------+---------+----------------------------------+
| name | key | label | type | description |
| | type | | | |
+------+-------+-------+---------+----------------------------------+
| crv | 2 | -1 | int / | EC Curve identifier - Taken from |
| | | | tstr | the COSE Curves registry |
| | | | | |
| x | 2 | -2 | bstr | X Coordinate |
| | | | | |
| y | 2 | -3 | bstr / | Y Coordinate |
| | | | bool | |
| | | | | |
| d | 2 | -4 | bstr | Private key |
+------+-------+-------+---------+----------------------------------+
Table 23: EC Key Parameters
13.2. Octet Key Pair
A new key type is defined for Octet Key Pairs (OKP). Do not assume
that keys using this type are elliptic curves. This key type could
be used for other curve types (for example, mathematics based on
hyper-elliptic surfaces).
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The key parameters defined in this section are summarized in
Table 24. The members that are defined for this key type are:
crv contains an identifier of the curve to be used with the key.
The curves defined in this document for this key type can be found
in Table 22. Other curves may be registered in the future and
private curves can be used as well.
x contains the x coordinate for the EC point. The octet string
represents a little-endian encoding of x.
d contains the private key.
For public keys, it is REQUIRED that 'crv' and 'x' be present in the
structure. For private keys, it is REQUIRED that 'crv' and 'd' be
present in the structure. For private keys, it is RECOMMENDED that
'x' also be present, but it can be recomputed from the required
elements and omitting it saves on space.
+------+------+-------+-------+-------------------------------------+
| name | key | label | type | description |
| | type | | | |
+------+------+-------+-------+-------------------------------------+
| crv | 1 | -1 | int / | EC Curve identifier - Taken from |
| | | | tstr | the COSE Key Common Parameters |
| | | | | registry |
| | | | | |
| x | 1 | -2 | bstr | X Coordinate |
| | | | | |
| d | 1 | -4 | bstr | Private key |
+------+------+-------+-------+-------------------------------------+
Table 24: Octet Key Pair Parameters
13.3. Symmetric Keys
Occasionally it is required that a symmetric key be transported
between entities. This key structure allows for that to happen.
For symmetric keys, the 'kty' member is set to 3 (Symmetric). The
member that is defined for this key type is:
k contains the value of the key.
This key structure does not have a form that contains only public
members. As it is expected that this key structure is going to be
transmitted, care must be taking that it is never transmitted
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accidentally or insecurely. For symmetric keys, it is REQUIRED that
'k' be present in the structure.
+------+----------+-------+------+-------------+
| name | key type | label | type | description |
+------+----------+-------+------+-------------+
| k | 4 | -1 | bstr | Key Value |
+------+----------+-------+------+-------------+
Table 25: Symmetric Key Parameters
14. CBOR Encoder Restrictions
There has been an attempt to limit the number of places where the
document needs to impose restrictions on how the CBOR Encoder needs
to work. We have managed to narrow it down to the following
restrictions:
o The restriction applies to the encoding the Sig_structure, the
Enc_structure, and the MAC_structure.
o The rules for Canonical CBOR (Section 3.9 of RFC 7049) MUST be
used in these locations. The main rule that needs to be enforced
is that all lengths in these structures MUST be encoded such that
they are encoded using definite lengths and the minimum length
encoding is used.
o Applications MUST NOT generate messages with the same label used
twice as a key in a single map. Applications MUST NOT parse and
process messages with the same label used twice as a key in a
single map. Applications can enforce the parse and process
requirement by using parsers that will fail the parse step or by
using parsers that will pass all keys to the application and the
application can perform the check for duplicate keys.
15. Application Profiling Considerations
This document is designed to provide a set of security services, but
not to provide implementation requirements for specific usage. The
interoperability requirements are provided for how each of the
individual services are used and how the algorithms are to be used
for interoperability. The requirements about which algorithms and
which services are needed are deferred to each application.
An example of a profile can be found in
[I-D.selander-ace-object-security] where two profiles are being
developed. One is for carrying content by itself, and the other is
for carrying content in combination with CoAP headers.
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It is intended that a profile of this document be created that
defines the interoperability requirements for that specific
application. This section provides a set of guidelines and topics
that need to be considered when profiling this document.
o Applications need to determine the set of messages defined in this
document that they will be using. The set of messages corresponds
fairly directly to the set of security services that are needed
and to the security levels needed.
o Applications may define new header parameters for a specific
purpose. Applications will often times select specific header
parameters to use or not to use. For example, an application
would normally state a preference for using either the IV or the
partial IV parameter. If the partial IV parameter is specified,
then the application would also need to define how the fixed
portion of the IV would be determined.
o When applications use externally defined authenticated data, they
need to define how that data is encoded. This document assumes
that the data will be provided as a byte stream. More information
can be found in Section 4.3.
o Applications need to determine the set of security algorithms that
are to be used. When selecting the algorithms to be used as the
mandatory to implement set, consideration should be given to
choosing different types of algorithms when two are chosen for a
specific purpose. An example of this would be choosing HMAC-
SHA512 and AES-CMAC as different MAC algorithms; the construction
is vastly different between these two algorithms. This means that
a weakening of one algorithm would be unlikely to lead to a
weakening of the other algorithms. Of course, these algorithms do
not provide the same level of security and thus may not be
comparable for the desired security functionality.
o Applications may need to provide some type of negotiation or
discovery method if multiple algorithms or message structures are
permitted. The method can be as simple as requiring
preconfiguration of the set of algorithms to providing a discovery
method built into the protocol. S/MIME provided a number of
different ways to approach the problem that applications could
follow:
* Advertising in the message (S/MIME capabilities) [RFC5751].
* Advertising in the certificate (capabilities extension)
[RFC4262].
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* Minimum requirements for the S/MIME, which have been updated
over time [RFC2633][RFC5751].
16. IANA Considerations
16.1. CBOR Tag assignment
It is requested that IANA assign the following tags from the "CBOR
Tags" registry. It is requested that the tags for COSE_Sign1,
COSE_Encrypt0, and COSE_Mac0 be assigned in the 1 to 23 value range
(one byte long when encoded). It is requested that the tags for
COSE_Sign, COSE_Encrypt and COSE_MAC be assigned in the 24 to 255
value range (two bytes long when encoded).
The tags to be assigned are in Table 1.
16.2. COSE Header Parameters Registry
It is requested that IANA create a new registry entitled "COSE Header
Parameters". The registry should be created as Expert Review
Required. Guidelines for the experts is provided Section 16.11. It
should be noted that in additional to the expert review, some
portions of the registry require a specification, potentially on
standards track, be supplied as well.
The columns of the registry are:
name The name is present to make it easier to refer to and discuss
the registration entry. The value is not used in the protocol.
Names are to be unique in the table.
label This is the value used for the label. The label can be either
an integer or a string. Registration in the table is based on the
value of the label requested. Integer values between 1 and 255
and strings of length 1 are designated as Standards Track Document
required. Integer values from 256 to 65535 and strings of length
2 are designated as Specification Required. Integer values of
greater than 65535 and strings of length greater than 2 are
designated as expert review. Integer values in the range -1 to
-65536 are delegated to the "COSE Header Algorithm Parameters"
registry. Integer values less than -65536 are marked as private
use.
value This contains the CBOR type for the value portion of the
label.
value registry This contains a pointer to the registry used to
contain values where the set is limited.
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description This contains a brief description of the header field.
specification This contains a pointer to the specification defining
the header field (where public).
The initial contents of the registry can be found in Table 2 and
Table 27. The specification column for all rows in that table should
be this document.
Additionally, the label of 0 is to be marked as 'Reserved'.
16.3. COSE Header Algorithm Parameters Registry
It is requested that IANA create a new registry entitled "COSE Header
Algorithm Parameters". The registry is to be created as Expert
Review Required. Expert review guidelines are provided in
Section 16.11.
The columns of the registry are:
name The name is present to make it easier to refer to and discuss
the registration entry. The value is not used in the protocol.
algorithm The algorithm(s) that this registry entry is used for.
This value is taken from the "COSE Algorithm Values" registry.
Multiple algorithms can be specified in this entry. For the
table, the algorithm, label pair MUST be unique.
label This is the value used for the label. The label is an integer
in the range of -1 to -65536.
value This contains the CBOR type for the value portion of the
label.
description This contains a brief description of the header field.
specification This contains a pointer to the specification defining
the header field (where public).
The initial contents of the registry can be found in Table 13,
Table 14, and Table 19. The specification column for all rows in
that table should be this document.
16.4. COSE Algorithms Registry
It is requested that IANA create a new registry entitled "COSE
Algorithms Registry". The registry is to be created as Expert Review
Required. Guidelines for the experts is provided Section 16.11. It
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should be noted that in additional to the expert review, some
portions of the registry require a specification, potentially on
standards track, be supplied as well.
The columns of the registry are:
value: The value to be used to identify this algorithm. Algorithm
values MUST be unique. The value can be a positive integer, a
negative integer or a string. Integer values between -256 and 255
and strings of length 1 are designated as Standards Track Document
required. Integer values from -65536 to 65535 and strings of
length 2 are designated as Specification Required. Integer values
of greater than 65535 and strings of length greater than 2 are
designated as expert review. Integer values less than -65536 are
marked as private use.
description: A short description of the algorithm.
specification: A document where the algorithm is defined (if
publicly available).
recommended: Does the IETF have a concensus recommendation to use
the algorithm. The legal values are 'yes', 'no' and 'deprecated'.
The initial contents of the registry can be found in Table 10,
Table 9, Table 11, Table 5, Table 7, Table 8, Table 15, Table 16,
Table 17, Table 6, Table 20 and Table 18. The specification column
for all rows in the table should be this document. The recommneded
column for all rows in the table are set to 'yes'.
Additionally, the label of 0 is to be marked as 'Reserved'.
NOTE: The assignment of algorithm identifiers in this document was
done so that positive numbers were used for the first layer objects
(COSE_Sign, COSE_Sign1, COSE_Encrypt, COSE_Encrypt0, COSE_Mac, and
COSE_Mac0). Negative numbers were used for second layer objects
(COSE_Signature and COSE_recipient). Expert reviewers should
consider this practice, but are not expected to be restricted by this
precedent.
16.5. COSE Key Common Parameters Registry
It is requested that IANA create a new registry entitled "COSE Key
Common Parameters" registry. The registry is to be created as Expert
Review Required. Guidelines for the experts is provided
Section 16.11. It should be noted that in additional to the expert
review, some portions of the registry require a specification,
potentially on standards track, be supplied as well.
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The columns of the registry are:
name This is a descriptive name that enables easier reference to the
item. It is not used in the encoding.
label The value to be used to identify this algorithm. Key map
labels MUST be unique. The label can be a positive integer, a
negative integer or a string. Integer values between 0 and 255
and strings of length 1 are designated as Standards Track Document
required. Integer values from 256 to 65535 and strings of length
2 are designated as Specification Required. Integer values of
greater than 65535 and strings of length greater than 2 are
designated as expert review. Integer values in the range -1 to
-65536 are used for key parameters specific to a single algorithm
delegated to the "COSE Key Type Parameter Labels" registry.
Integer values less than -65536 are marked as private use.
CBOR Type This field contains the CBOR type for the field.
registry This field denotes the registry that values come from, if
one exists.
description This field contains a brief description for the field.
specification This contains a pointer to the public specification
for the field if one exists
This registry will be initially populated by the values in Table 3.
The specification column for all of these entries will be this
document.
16.6. COSE Key Type Parameters Registry
It is requested that IANA create a new registry "COSE Key Type
Parameters". The registry is to be created as Expert Review
Required. Expert review guidelines are provided in Section 16.11.
The columns of the table are:
key type This field contains a descriptive string of a key type.
This should be a value that is in the COSE Key Common Parameters
table and is placed in the 'kty' field of a COSE Key structure.
name This is a descriptive name that enables easier reference to the
item. It is not used in the encoding.
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label The label is to be unique for every value of key type. The
range of values is from -256 to -1. Labels are expected to be
reused for different keys.
CBOR type This field contains the CBOR type for the field.
description This field contains a brief description for the field.
specification This contains a pointer to the public specification
for the field if one exists.
This registry will be initially populated by the values in Table 23,
Table 24, and Table 25. The specification column for all of these
entries will be this document.
16.7. COSE Key Type Registry
It is requested that IANA create a new registry "COSE Key Type
Registry". The registry is to be created as Expert Review Required.
Expert review guidelines are provided in Section 16.11.
The columns of this table are:
name This is a descriptive name that enables easier reference to the
item. The name MUST be unique. It is not used in the encoding.
value This is the value used to identify the curve. These values
MUST be unique. The value can be a positive integer, a negative
integer or a string.
description This field contains a brief description of the curve.
specification This contains a pointer to the public specification
for the curve if one exists.
This registry will be initially populated by the values in Table 21.
The specification column for all of these entries will be this
document.
16.8. COSE Elliptic Curve Parameters Registry
It is requested that IANA create a new registry "COSE Elliptic Curve
Parameters". The registry is to be created as Expert Review
Required. Guidelines for the experts is provided Section 16.11. It
should be noted that in additional to the expert review, some
portions of the registry require a specification, potentially on
standards track, be supplied as well.
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The columns of the table are:
name This is a descriptive name that enables easier reference to the
item. It is not used in the encoding.
value This is the value used to identify the curve. These values
MUST be unique. The integer values from -256 to 255 are
designated as Standards Track Document Required. The integer
values from 256 to 65535 and -65536 to -257 are designated as
Specification Required. Integer values over 65535 are designated
as expert review. Integer values less than -65536 are marked as
private use.
key type This designates the key type(s) that can be used with this
curve.
description This field contains a brief description of the curve.
specification This contains a pointer to the public specification
for the curve if one exists.
recommended: Does the IETF have a concensus recommendation to use
the algorithm. The legal values are 'yes', 'no' and 'deprecated'.
This registry will be initially populated by the values in Table 22.
The specification column for all of these entries will be this
document. The recommended column for all of the inital entries will
be 'yes'.
16.9. Media Type Registrations
16.9.1. COSE Security Message
This section registers the "application/cose" media type in the
"Media Types" registry. These media types are used to indicate that
the content is a COSE message.
Type name: application
Subtype name: cose
Required parameters: N/A
Optional parameters: cose-type
Encoding considerations: binary
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Security considerations: See the Security Considerations section
of RFC TBD.
Interoperability considerations: N/A
Published specification: RFC TBD
Applications that use this media type: IoT applications sending
security content over HTTP(S) transports.
Fragment identifier considerations: N/A
Additional information:
* Magic number(s): N/A
* File extension(s): cbor
* Macintosh file type code(s): N/A
Person & email address to contact for further information:
iesg@ietf.org
Intended usage: COMMON
Restrictions on usage: N/A
Author: Jim Schaad, ietf@augustcellars.com
Change Controller: IESG
Provisional registration? No
16.9.2. COSE Key media type
This section registers the "application/cose-key" and "application/
cose-key-set" media types in the "Media Types" registry. These media
types are used to indicate, respectively, that content is a COSE_Key
or COSE_KeySet object.
The template for registering "application/cose-key" is:
Type name: application
Subtype name: cose-key
Required parameters: N/A
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Optional parameters: N/A
Encoding considerations: binary
Security considerations: See the Security Considerations section
of RFC TBD.
Interoperability considerations: N/A
Published specification: RFC TBD
Applications that use this media type: Distribution of COSE based
keys for IoT applications.
Fragment identifier considerations: N/A
Additional information:
* Magic number(s): N/A
* File extension(s): cbor
* Macintosh file type code(s): N/A
Person & email address to contact for further information:
iesg@ietf.org
Intended usage: COMMON
Restrictions on usage: N/A
Author: Jim Schaad, ietf@augustcellars.com
Change Controller: IESG
Provisional registration? No
The template for registering "application/cose-key-set" is:
Type name: application
Subtype name: cose-key-set
Required parameters: N/A
Optional parameters: N/A
Encoding considerations: binary
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Security considerations: See the Security Considerations section
of RFC TBD.
Interoperability considerations: N/A
Published specification: RFC TBD
Applications that use this media type: Distribution of COSE based
keys for IoT applications.
Fragment identifier considerations: N/A
Additional information:
* Magic number(s): N/A
* File extension(s): cbor
* Macintosh file type code(s): N/A
Person & email address to contact for further information:
iesg@ietf.org
Intended usage: COMMON
Restrictions on usage: N/A
Author: Jim Schaad, ietf@augustcellars.com
Change Controller: IESG
Provisional registration? No
16.10. CoAP Content-Format Registrations
IANA is requested to add the following entries to the "CoAP Content-
Format" registry. ID assignment in the 24-255 range is requested.
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+---------------------------------+----------+-------+--------------+
| Media Type | Encoding | ID | Reference |
+---------------------------------+----------+-------+--------------+
| application/cose; cose-type | | TBD10 | [This |
| ="cose-sign" | | | Document] |
| | | | |
| application/cose; cose-type | | TBD11 | [This |
| ="cose-sign1" | | | Document] |
| | | | |
| application/cose; cose-type | | TBD12 | [This |
| ="cose-encrypt" | | | Document] |
| | | | |
| application/cose; cose-type | | TBD13 | [This |
| ="cose-encrypt0" | | | Document] |
| | | | |
| application/cose; cose-type | | TBD14 | [This |
| ="cose-mac" | | | Document] |
| | | | |
| application/cose; cose-type | | TBD15 | [This |
| ="cose-mac0" | | | Document] |
| | | | |
| application/cose-key | | TBD16 | [This |
| | | | Document] |
| | | | |
| application/cose-key-set | | TBD17 | [This |
| | | | Document |
+---------------------------------+----------+-------+--------------+
Table 26
16.11. Expert Review Instructions
All of the IANA registries established in this document are defined
as expert review. This section gives some general guidelines for
what the experts should be looking for, but they are being designated
as experts for a reason so they should be given substantial latitude.
Expert reviewers should take into consideration the following points:
o Point squatting should be discouraged. Reviewers are encouraged
to get sufficient information for registration requests to ensure
that the usage is not going to duplicate one that is already
registered and that the point is likely to be used in deployments.
The zones tagged as private use are intended for testing purposes
and closed environments, code points in other ranges should not be
assigned for testing.
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o Specifications are required for the standards track range of point
assignment. Specifications should exist for specification
required ranges, but early assignment before a specification is
available is considered to be permissible. Specifications are
needed for the first-come, first-serve range if they are expected
to be used outside of closed environments in an interoperable way.
When specifications are not provided, the description provided
needs to have sufficient information to identify what the point is
being used for.
o Experts should take into account the expected usage of fields when
approving point assignment. The fact that there is a range for
standards track documents does not mean that a standards track
document cannot have points assigned outside of that range. The
length of the encoded value should be weighed against how many
code points of that length are left, the size of device it will be
used on, and the number of code points left that encode to that
size.
o When algorithms are registered, vanity registrations should be
discouraged. One way to do this is to require registrations to
provide additional documentation on security analysis of the
algorithm. Another thing that should be considered is to request
for an opinion on the algorithm from the Crypto Forum Research
Group (CFRG). Algorithms that do not meet the security
requirements of the community and the messages structures should
not be registered.
17. Implementation Status
This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this
Internet-Draft, and is based on a proposal described in [RFC7942].
The description of implementations in this section is intended to
assist the IETF in its decision processes in progressing drafts to
RFCs. Please note that the listing of any individual implementation
here does not imply endorsement by the IETF. Furthermore, no effort
has been spent to verify the information presented here that was
supplied by IETF contributors. This is not intended as, and must not
be construed to be, a catalog of available implementations or their
features. Readers are advised to note that other implementations may
exist.
According to [RFC7942], "this will allow reviewers and working groups
to assign due consideration to documents that have the benefit of
running code, which may serve as evidence of valuable experimentation
and feedback that have made the implemented protocols more mature.
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It is up to the individual working groups to use this information as
they see fit".
17.1. Author's Versions
There are three different implementations that have been created by
the author of the document both to create the examples that are
included in the document and to validate the structures and
methodology used in the design of COSE.
Implementation Location: https://github.com/cose-wg
Primary Maintainer: Jim Schaad
Languages: There are three different languages that are currently
supported: Java, C# and C.
Cryptography: The Java and C# libraries use Bouncy Castle to
provide the required cryptography. The C version uses OPENSSL
Version 1.0 for the cryptography.
Coverage: The libraries currently do not have full support for
counter signatures of either variety. They do have support to
allow for implicit algorithm support as they allow for the
application to set attributes that are not to be sent in the
message.
Testing: All of the examples in the example library are generated
by the C# library and then validated using the Java and C
libraries. All three libraries have tests to allow for the
creating of the same messages that are in the example library
followed by validating them. These are not compared against the
example library. The Java and C# libraries have unit testing
included. Not all of the MUST statements in the document have
been implemented as part of the libraries. One such statement is
the requirement that unique labels be present.
Licensing: Revised BSD License
17.2. COSE Testing Library
Implementation Location: https://github.com/cose-wg/Examples
Primary Maintainer: Jim Schaad
Description: A set of tests for the COSE library is provided as
part of the implementation effort. Both success and fail tests
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have been provided. All of the examples in this document are part
of this example set.
Coverage: An attempt has been made to have test cases for every
message type and algorithm in the document. Currently examples
dealing with counter signatures, EdDSA, and ECDH with Curve24459
and Goldilocks are missing.
Licensing: Public Domain
18. Security Considerations
There are a number of security considerations that need to be taken
into account by implementers of this specification. The security
considerations that are specific to an individual algorithm are
placed next to the description of the algorithm. While some
considerations have been highlighted here, additional considerations
may be found in the documents listed in the references.
Implementations need to protect the private key material for any
individuals. There are some cases in this document that need to be
highlighted on this issue.
o Using the same key for two different algorithms can leak
information about the key. It is therefore recommended that keys
be restricted to a single algorithm.
o Use of 'direct' as a recipient algorithm combined with a second
recipient algorithm, exposes the direct key to the second
recipient.
o Several of the algorithms in this document have limits on the
number of times that a key can be used without leaking information
about the key.
The use of ECDH and direct plus KDF (with no key wrap) will not
directly lead to the private key being leaked; the one way function
of the KDF will prevent that. There is however, a different issue
that needs to be addressed. Having two recipients requires that the
CEK be shared between two recipients. The second recipient therefore
has a CEK that was derived from material that can be used for the
weak proof of origin. The second recipient could create a message
using the same CEK and send it to the first recipient, the first
recipient would, for either static-static ECDH or direct plus KDF,
make an assumption that the CEK could be used for proof of origin
even though it is from the wrong entity. If the key wrap step is
added, then no proof of origin is implied and this is not an issue.
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Although it has been mentioned before, the use of a single key for
multiple algorithms has been demonstrated in some cases to leak
information about a key, provide for attackers to forge integrity
tags, or gain information about encrypted content. Binding a key to
a single algorithm prevents these problems. Key creators and key
consumers are strongly encouraged not only to create new keys for
each different algorithm, but to include that selection of algorithm
in any distribution of key material and strictly enforce the matching
of algorithms in the key structure to algorithms in the message
structure. In addition to checking that algorithms are correct, the
key form needs to be checked as well. Do not use an 'EC2' key where
an 'OKP' key is expected.
Before using a key for transmission, or before acting on information
received, a trust decision on a key needs to be made. Is the data or
action something that the entity associated with the key has a right
to see or a right to request? A number of factors are associated
with this trust decision. Some of the ones that are highlighted here
are:
o What are the permissions associated with the key owner?
o Is the cryptographic algorithm acceptable in the current context?
o Have the restrictions associated with the key, such as algorithm
or freshness, been checked and are correct?
o Is the request something that is reasonable, given the current
state of the application?
o Have any security considerations that are part of the message been
enforced (as specified by the application or 'crit' parameter)?
There are a large number of algorithms presented in this document
that use nonce values. For all of the nonces defined in this
document, there is some type of restriction on the nonce being a
unique value either for a key or for some other conditions. In all
of these cases, there is no known requirement on the nonce being both
unique and unpredictable, under these circumstances it reasonable to
use a counter for creation of the nonce. In cases where one wants
the pattern of the nonce to be unpredictable as well as unique, one
can use a key created for that purpose and encrypt the counter to
produce the nonce value.
One area that has been starting to get exposure is doing traffic
analysis of encrypted messages based on the length of the message.
This specification does not provide for a uniform method of providing
padding as part of the message structure. An observer can
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distinguish between two different strings (for example, 'YES' and
'NO') based on length for all of the content encryption algorithms
that are defined in this document. This means that it is up to
applications to document how content padding is to be done in order
to prevent or discourage such analysis. (For example, the strings
could be defined as 'YES' and 'NO '.)
19. References
19.1. Normative References
[AES-GCM] Dworkin, M., "NIST Special Publication 800-38D:
Recommendation for Block Cipher Modes of Operation:
Galois/Counter Mode (GCM) and GMAC.", Nov 2007.
[COAP.Formats]
IANA, , "CoAP Content-Formats".
[DSS] U.S. National Institute of Standards and Technology,
"Digital Signature Standard (DSS)", July 2013.
[I-D.irtf-cfrg-eddsa]
Josefsson, S. and I. Liusvaara, "Edwards-curve Digital
Signature Algorithm (EdDSA)", draft-irtf-cfrg-eddsa-08
(work in progress), August 2016.
[MAC] NiST, N., "FIPS PUB 113: Computer Data Authentication",
May 1985.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
DOI 10.17487/RFC2104, February 1997,
<http://www.rfc-editor.org/info/rfc2104>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC3394] Schaad, J. and R. Housley, "Advanced Encryption Standard
(AES) Key Wrap Algorithm", RFC 3394, DOI 10.17487/RFC3394,
September 2002, <http://www.rfc-editor.org/info/rfc3394>.
[RFC3610] Whiting, D., Housley, R., and N. Ferguson, "Counter with
CBC-MAC (CCM)", RFC 3610, DOI 10.17487/RFC3610, September
2003, <http://www.rfc-editor.org/info/rfc3610>.
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[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869,
DOI 10.17487/RFC5869, May 2010,
<http://www.rfc-editor.org/info/rfc5869>.
[RFC6090] McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
Curve Cryptography Algorithms", RFC 6090,
DOI 10.17487/RFC6090, February 2011,
<http://www.rfc-editor.org/info/rfc6090>.
[RFC6979] Pornin, T., "Deterministic Usage of the Digital Signature
Algorithm (DSA) and Elliptic Curve Digital Signature
Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August
2013, <http://www.rfc-editor.org/info/rfc6979>.
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
October 2013, <http://www.rfc-editor.org/info/rfc7049>.
[RFC7539] Nir, Y. and A. Langley, "ChaCha20 and Poly1305 for IETF
Protocols", RFC 7539, DOI 10.17487/RFC7539, May 2015,
<http://www.rfc-editor.org/info/rfc7539>.
[RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
for Security", RFC 7748, DOI 10.17487/RFC7748, January
2016, <http://www.rfc-editor.org/info/rfc7748>.
[SEC1] Standards for Efficient Cryptography Group, "SEC 1:
Elliptic Curve Cryptography", May 2009.
19.2. Informative References
[I-D.greevenbosch-appsawg-cbor-cddl]
Vigano, C. and H. Birkholz, "CBOR data definition language
(CDDL): a notational convention to express CBOR data
structures", draft-greevenbosch-appsawg-cbor-cddl-09 (work
in progress), September 2016.
[I-D.moriarty-pkcs1]
Moriarty, K., Kaliski, B., Jonsson, J., and A. Rusch,
"PKCS #1 Version 2.2: RSA Cryptography Specifications",
draft-moriarty-pkcs1-03 (work in progress), September
2016.
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[I-D.moriarty-pkcs5-v2dot1]
Moriarty, K., Kaliski, B., and A. Rusch, "PKCS #5:
Password-Based Cryptography Specification Version 2.1",
draft-moriarty-pkcs5-v2dot1-04 (work in progress),
September 2016.
[I-D.selander-ace-object-security]
Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security of CoAP (OSCOAP)", draft-selander-ace-
object-security-06 (work in progress), October 2016.
[PVSig] Brown, D. and D. Johnson, "Formal Security Proofs for a
Signature Scheme with Partial Message Recover", February
2000.
[RFC2633] Ramsdell, B., Ed., "S/MIME Version 3 Message
Specification", RFC 2633, DOI 10.17487/RFC2633, June 1999,
<http://www.rfc-editor.org/info/rfc2633>.
[RFC4231] Nystrom, M., "Identifiers and Test Vectors for HMAC-SHA-
224, HMAC-SHA-256, HMAC-SHA-384, and HMAC-SHA-512",
RFC 4231, DOI 10.17487/RFC4231, December 2005,
<http://www.rfc-editor.org/info/rfc4231>.
[RFC4262] Santesson, S., "X.509 Certificate Extension for Secure/
Multipurpose Internet Mail Extensions (S/MIME)
Capabilities", RFC 4262, DOI 10.17487/RFC4262, December
2005, <http://www.rfc-editor.org/info/rfc4262>.
[RFC4493] Song, JH., Poovendran, R., Lee, J., and T. Iwata, "The
AES-CMAC Algorithm", RFC 4493, DOI 10.17487/RFC4493, June
2006, <http://www.rfc-editor.org/info/rfc4493>.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
<http://www.rfc-editor.org/info/rfc4949>.
[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
<http://www.rfc-editor.org/info/rfc5116>.
[RFC5480] Turner, S., Brown, D., Yiu, K., Housley, R., and T. Polk,
"Elliptic Curve Cryptography Subject Public Key
Information", RFC 5480, DOI 10.17487/RFC5480, March 2009,
<http://www.rfc-editor.org/info/rfc5480>.
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[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, DOI 10.17487/RFC5652, September 2009,
<http://www.rfc-editor.org/info/rfc5652>.
[RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2 Message
Specification", RFC 5751, DOI 10.17487/RFC5751, January
2010, <http://www.rfc-editor.org/info/rfc5751>.
[RFC5752] Turner, S. and J. Schaad, "Multiple Signatures in
Cryptographic Message Syntax (CMS)", RFC 5752,
DOI 10.17487/RFC5752, January 2010,
<http://www.rfc-editor.org/info/rfc5752>.
[RFC5990] Randall, J., Kaliski, B., Brainard, J., and S. Turner,
"Use of the RSA-KEM Key Transport Algorithm in the
Cryptographic Message Syntax (CMS)", RFC 5990,
DOI 10.17487/RFC5990, September 2010,
<http://www.rfc-editor.org/info/rfc5990>.
[RFC6151] Turner, S. and L. Chen, "Updated Security Considerations
for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
RFC 6151, DOI 10.17487/RFC6151, March 2011,
<http://www.rfc-editor.org/info/rfc6151>.
[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC6838, January 2013,
<http://www.rfc-editor.org/info/rfc6838>.
[RFC7159] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
2014, <http://www.rfc-editor.org/info/rfc7159>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<http://www.rfc-editor.org/info/rfc7252>.
[RFC7515] Jones, M., Bradley, J., and N. Sakimura, "JSON Web
Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
2015, <http://www.rfc-editor.org/info/rfc7515>.
[RFC7516] Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)",
RFC 7516, DOI 10.17487/RFC7516, May 2015,
<http://www.rfc-editor.org/info/rfc7516>.
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[RFC7517] Jones, M., "JSON Web Key (JWK)", RFC 7517,
DOI 10.17487/RFC7517, May 2015,
<http://www.rfc-editor.org/info/rfc7517>.
[RFC7518] Jones, M., "JSON Web Algorithms (JWA)", RFC 7518,
DOI 10.17487/RFC7518, May 2015,
<http://www.rfc-editor.org/info/rfc7518>.
[RFC7942] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", BCP 205,
RFC 7942, DOI 10.17487/RFC7942, July 2016,
<http://www.rfc-editor.org/info/rfc7942>.
[SP800-56A]
Barker, E., Chen, L., Roginsky, A., and M. Smid, "NIST
Special Publication 800-56A: Recommendation for Pair-Wise
Key Establishment Schemes Using Discrete Logarithm
Cryptography", May 2013.
[W3C.WebCrypto]
Watson, M., "Web Cryptography API", July 2016.
Appendix A. Guidelines for External Data Authentication of Algorithms
There has been a portion of the working group who have expressed a
strong desire to relax the rule that the algorithm identifier be
required to appear in each level of a COSE object. There are two
basic reasons that have been advanced to support this position.
First, the resulting message will be smaller if the algorithm
identifier is omitted from the most common messages in a CoAP
environment. Second, there is a potential bug that will arise if
full checking is not done correctly between the different places that
an algorithm identifier could be placed (the message itself, an
application statement, the key structure that the sender possesses
and the key structure the recipient possesses).
This appendix lays out how such a change can be made and the details
that an application needs to specify in order to use this option.
Two different sets of details are specified: Those needed to omit an
algorithm identifier and those needed to use a variant on the counter
signature attribute that contains no attributes about itself.
A.1. Algorithm Identification
In this section are laid out three sets of recommendations. The
first set of recommendations apply to having an implicit algorithm
identified for a single layer of a COSE object. The second set of
recommendations apply to having multiple implicit algorithms
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identified for multiple layers of a COSE object. The third set of
recommendations apply to having implicit algorithms for multiple COSE
object constructs.
RFC 2119 language is deliberately not used here. This specification
can provide recommendations, but it cannot enforce them.
This set of recommendations applies to the case where an application
is distributing a fixed algorithm along with the key information for
use in a single COSE object. This normally applies to the smallest
of the COSE objects, specifically COSE_Sign1, COSE_Mac0, and
COSE_Encrypt0, but could apply to the other structures as well.
The following items should be taken into account:
o Applications need to list the set of COSE structures that implicit
algorithms are to be used in. Applications need to require that
the receipt of an explicit algorithm identifier in one of these
structures will lead to the message being rejected. This
requirement is stated so that there will never be a case where
there is any ambiguity about the question of which algorithm
should be used, the implicit or the explicit one. This applies
even if the transported algorithm identifier is a protected
attribute. This applies even if the transported algorithm is the
same as the implicit algorithm.
o Applications need to define the set of information that is to be
considered to be part of a context when omitting algorithm
identifiers. At a minimum, this would be the key identifier (if
needed), the key, the algorithm, and the COSE structure it is used
with. Applications should restrict the use of a single key to a
single algorithm. As noted for some of the algorithms in this
document, the use of the same key in different related algorithms
can lead to leakage of information about the key, leakage about
the data or the ability to perform forgeries.
o In many cases, applications that make the algorithm identifier
implicit will also want to make the context identifier implicit
for the same reason. That is, omitting the context identifier
will decrease the message size (potentially significantly
depending on the length of the identifier). Applications that do
this will need to describe the circumstances where the context
identifier is to be omitted and how the context identifier is to
be inferred in these cases. (Exhaustive search over all of the
keys would normally not be considered to be acceptable.) An
example of how this can be done is to tie the context to a
transaction identifier. Both would be sent on the original
message, but only the transaction identifier would need to be sent
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after that point as the context is tied into the transaction
identifier. Another way would be to associate a context with a
network address. All messages coming from a single network
address can be assumed to be associated with a specific context.
(In this case the address would normally be distributed as part of
the context.)
o Applications cannot rely on key identifiers being unique unless
they take significant efforts to ensure that they are computed in
such a way as to create this guarantee. Even when an application
does this, the uniqueness might be violated if the application is
run in different contexts (i.e., with a different context
provider) or if the system combines the security contexts from
different applications together into a single store.
o Applications should continue the practice of protecting the
algorithm identifier. Since this is not done by placing it in the
protected attributes field, applications should define an
application specific external data structure that includes this
value. This external data field can be used as such for content
encryption, MAC, and signature algorithms. It can be used in the
SuppPrivInfo field for those algorithms which use a KDF function
to derive a key value. Applications may also want to protect
other information that is part of the context structure as well.
It should be noted that those fields, such as the key or a base
IV, are protected by virtue of being used in the cryptographic
computation and do not need to be included in the external data
field.
The second case is having multiple implicit algorithm identifiers
specified for a multiple layer COSE object. An example of how this
would work is the encryption context that an application specifies
contains a content encryption algorithm, a key wrap algorithm, a key
identifier, and a shared secret. The sender omits sending the
algorithm identifier for both the content layer and the recipient
layer leaving only the key identifier. The receiver then uses the
key identifier to get the implicit algorithm identifiers.
The following additional items need to be taken into consideration:
o Applications that want to support this will need to define a
structure that allows for, and clearly identifies, both the COSE
structure to be used with a given key and the structure and
algorithm to be used for the secondary layer. The key for the
secondary layer is computed per normal from the recipient layer.
The third case is having multiple implicit algorithm identifiers, but
targeted at potentially unrelated layers or different COSE objects.
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There are a number of different scenarios where this might be
applicable. Some of these scenarios are:
o Two contexts are distributed as a pair. Each of the contexts is
for use with a COSE_Encrypt message. Each context will consist of
distinct secret keys and IVs and potentially even different
algorithms. One context is for sending messages from party A to
party B, the second context is for sending messages from party B
to party A. This means that there is no chance for a reflection
attack to occur as each party uses different secret keys to send
its messages, a message that is reflected back to it would fail to
decrypt.
o Two contexts are distributed as a pair. The first context is used
for encryption of the message; the second context is used to place
a counter signature on the message. The intention is that the
second context can be distributed to other entities independently
of the first context. This allows these entities to validate that
the message came from an individual without being able to decrypt
the message and see the content.
o Two contexts are distributed as a pair. The first context
contains a key for dealing with MACed messages, the second context
contains a key for dealing with encrypted messages. This allows
for a unified distribution of keys to participants for different
types of messages that have different keys, but where the keys may
be used in coordinated manner.
For these cases, the following additional items need to be
considered:
o Applications need to ensure that the multiple contexts stay
associated. If one of the contexts is invalidated for any reason,
all of the contexts associated with it should also be invalidated.
A.2. Counter Signature Without Headers
There is a group of people who want to have a counter signature
parameter that is directly tied to the value being signed and thus
the authenticated and unauthenticated buckets can be removed from the
message being sent. The focus on this is an even smaller size, as
all of the information on the process of creating the counter
signature is implicit rather than being explicitly carried in the
message. This includes not only the algorithm identifier as
presented above, but also items such as the key identification is
always external to the signature structure. This means that the
entities that are doing the validation of the counter signature are
required to infer which key is to be used from context rather than
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being explicit. One way of doing this would be to presume that all
data coming from a specific port (or to a specific URL) is to be
validated by a specific key. (Note that this does not require that
the key identifier be part of the value signed as it does not serve a
cryptographic purpose. If the key validates the counter signature,
then it should be presumed that the entity associated with that key
produced the signature.)
When computing the signature for the bare counter signature header,
the same Sig_structure defined in Section 4.4 is used. The
sign_protected field is omitted, as there is no protected header
field in in this counter signature header. The value of
"CounterSignature0" is placed in the context field of the
Sig_stucture.
+-------------------+-------+-------+-------+-----------------------+
| name | label | value | value | description |
| | | type | | |
+-------------------+-------+-------+-------+-----------------------+
| CounterSignature0 | 9 | bstr | | Counter signature |
| | | | | with implied signer |
| | | | | and headers |
+-------------------+-------+-------+-------+-----------------------+
Table 27
Appendix B. Two Layers of Recipient Information
All of the currently defined recipient algorithms classes only use
two layers of the COSE_Encrypt structure. The first layer is the
message content and the second layer is the content key encryption.
However, if one uses a recipient algorithm such as RSA-KEM (see
Appendix A of RSA-KEM [RFC5990]), then it makes sense to have three
layers of the COSE_Encrypt structure.
These layers would be:
o Layer 0: The content encryption layer. This layer contains the
payload of the message.
o Layer 1: The encryption of the CEK by a KEK.
o Layer 2: The encryption of a long random secret using an RSA key
and a key derivation function to convert that secret into the KEK.
This is an example of what a triple layer message would look like.
The message has the following layers:
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o Layer 0: Has a content encrypted with AES-GCM using a 128-bit key.
o Layer 1: Uses the AES Key wrap algorithm with a 128-bit key.
o Layer 2: Uses ECDH Ephemeral-Static direct to generate the layer 1
key.
In effect, this example is a decomposed version of using the ECDH-
ES+A128KW algorithm.
Size of binary file is 183 bytes
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96(
[
/ protected / h'a10101' / {
\ alg \ 1:1 \ AES-GCM 128 \
} / ,
/ unprotected / {
/ iv / 5:h'02d1f7e6f26c43d4868d87ce'
},
/ ciphertext / h'64f84d913ba60a76070a9a48f26e97e863e2852948658f0
811139868826e89218a75715b',
/ recipients / [
[
/ protected / h'',
/ unprotected / {
/ alg / 1:-3 / A128KW /
},
/ ciphertext / h'dbd43c4e9d719c27c6275c67d628d493f090593db82
18f11',
/ recipients / [
[
/ protected / h'a1013818' / {
\ alg \ 1:-25 \ ECDH-ES + HKDF-256 \
} / ,
/ unprotected / {
/ ephemeral / -1:{
/ kty / 1:2,
/ crv / -1:1,
/ x / -2:h'b2add44368ea6d641f9ca9af308b4079aeb519f11
e9b8a55a600b21233e86e68',
/ y / -3:false
},
/ kid / 4:'meriadoc.brandybuck@buckland.example'
},
/ ciphertext / h''
]
]
]
]
]
)
Appendix C. Examples
This appendix includes a set of examples that show the different
features and message types that have been defined in this document.
To make the examples easier to read, they are presented using the
extended CBOR diagnostic notation (defined in
[I-D.greevenbosch-appsawg-cbor-cddl]) rather than as a binary dump.
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A GitHub project has been created at https://github.com/cose-wg/
Examples that contains not only the examples presented in this
document, but a more complete set of testing examples as well. Each
example is found in a JSON file that contains the inputs used to
create the example, some of the intermediate values that can be used
in debugging the example and the output of the example presented in
both a hex and a CBOR diagnostic notation format. Some of the
examples at the site are designed failure testing cases; these are
clearly marked as such in the JSON file. If errors in the examples
in this document are found, the examples on github will be updated
and a note to that effect will be placed in the JSON file.
As noted, the examples are presented using the CBOR's diagnostic
notation. A Ruby based tool exists that can convert between the
diagnostic notation and binary. This tool can be installed with the
command line:
gem install cbor-diag
The diagnostic notation can be converted into binary files using the
following command line:
diag2cbor.rb < inputfile > outputfile
The examples can be extracted from the XML version of this document
via an XPath expression as all of the artwork is tagged with the
attribute type='CBORdiag'. (Depending on the XPath evaluator one is
using, it may be necessary to deal with > as an entity.)
//artwork[@type='CDDL']/text()
C.1. Examples of Signed Message
C.1.1. Single Signature
This example uses the following:
o Signature Algorithm: ECDSA w/ SHA-256, Curve P-256
Size of binary file is 103 bytes
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[
/ protected / h'',
/ unprotected / {},
/ payload / 'This is the content.',
/ signatures / [
[
/ protected / h'a10126' / {
\ alg \ 1:-7 \ ECDSA 256 \
} / ,
/ unprotected / {
/ kid / 4:'11'
},
/ signature / h'e2aeafd40d69d19dfe6e52077c5d7ff4e408282cbefb
5d06cbf414af2e19d982ac45ac98b8544c908b4507de1e90b717c3d34816fe926a2b
98f53afd2fa0f30a'
]
]
]
)
C.1.2. Multiple Signers
This example uses the following:
o Signature Algorithm: ECDSA w/ SHA-256, Curve P-256
o Signature Algorithm: ECDSA w/ SHA-512, Curve P-521
Size of binary file is 277 bytes
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[
/ protected / h'',
/ unprotected / {},
/ payload / 'This is the content.',
/ signatures / [
[
/ protected / h'a10126' / {
\ alg \ 1:-7 \ ECDSA 256 \
} / ,
/ unprotected / {
/ kid / 4:'11'
},
/ signature / h'e2aeafd40d69d19dfe6e52077c5d7ff4e408282cbefb
5d06cbf414af2e19d982ac45ac98b8544c908b4507de1e90b717c3d34816fe926a2b
98f53afd2fa0f30a'
],
[
/ protected / h'a1013823' / {
\ alg \ 1:-36
} / ,
/ unprotected / {
/ kid / 4:'bilbo.baggins@hobbiton.example'
},
/ signature / h'00a2d28a7c2bdb1587877420f65adf7d0b9a06635dd1
de64bb62974c863f0b160dd2163734034e6ac003b01e8705524c5c4ca479a952f024
7ee8cb0b4fb7397ba08d009e0c8bf482270cc5771aa143966e5a469a09f613488030
c5b07ec6d722e3835adb5b2d8c44e95ffb13877dd2582866883535de3bb03d01753f
83ab87bb4f7a0297'
]
]
]
)
C.1.3. Counter Signature
This example uses the following:
o Signature Algorithm: ECDSA w/ SHA-256, Curve P-256
o The same parameters are used for both the signature and the
counter signature.
Size of binary file is 180 bytes
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[
/ protected / h'',
/ unprotected / {
/ countersign / 7:[
/ protected / h'a10126' / {
\ alg \ 1:-7 \ ECDSA 256 \
} / ,
/ unprotected / {
/ kid / 4:'11'
},
/ signature / h'5ac05e289d5d0e1b0a7f048a5d2b643813ded50bc9e4
9220f4f7278f85f19d4a77d655c9d3b51e805a74b099e1e085aacd97fc29d72f887e
8802bb6650cceb2c'
]
},
/ payload / 'This is the content.',
/ signatures / [
[
/ protected / h'a10126' / {
\ alg \ 1:-7 \ ECDSA 256 \
} / ,
/ unprotected / {
/ kid / 4:'11'
},
/ signature / h'e2aeafd40d69d19dfe6e52077c5d7ff4e408282cbefb
5d06cbf414af2e19d982ac45ac98b8544c908b4507de1e90b717c3d34816fe926a2b
98f53afd2fa0f30a'
]
]
]
)
C.1.4. Signature w/ Criticality
This example uses the following:
o Signature Algorithm: ECDSA w/ SHA-256, Curve P-256
o There is a criticality marker on the "reserved" header parameter
Size of binary file is 125 bytes
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[
/ protected / h'a2687265736572766564f40281687265736572766564' /
{
"reserved":false,
\ crit \ 2:[
"reserved"
]
} / ,
/ unprotected / {},
/ payload / 'This is the content.',
/ signatures / [
[
/ protected / h'a10126' / {
\ alg \ 1:-7 \ ECDSA 256 \
} / ,
/ unprotected / {
/ kid / 4:'11'
},
/ signature / h'3fc54702aa56e1b2cb20284294c9106a63f91bac658d
69351210a031d8fc7c5ff3e4be39445b1a3e83e1510d1aca2f2e8a7c081c7645042b
18aba9d1fad1bd9c'
]
]
]
)
C.2. Single Signer Examples
C.2.1. Single ECDSA signature
This example uses the following:
o Signature Algorithm: ECDSA w/ SHA-256, Curve P-256
Size of binary file is 98 bytes
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[
/ protected / h'a10126' / {
\ alg \ 1:-7 \ ECDSA 256 \
} / ,
/ unprotected / {
/ kid / 4:'11'
},
/ payload / 'This is the content.',
/ signature / h'eae868ecc176883766c5dc5ba5b8dca25dab3c2e56a551ce
5705b793914348e19f43d6c6ba654472da301b645b293c9ba939295b97c4bdb84778
2bff384c5794'
]
)
C.3. Examples of Enveloped Messages
C.3.1. Direct ECDH
This example uses the following:
o CEK: AES-GCM w/ 128-bit key
o Recipient class: ECDH Ephemeral-Static, Curve P-256
Size of binary file is 151 bytes
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[
/ protected / h'a10101' / {
\ alg \ 1:1 \ AES-GCM 128 \
} / ,
/ unprotected / {
/ iv / 5:h'c9cf4df2fe6c632bf7886413'
},
/ ciphertext / h'7adbe2709ca818fb415f1e5df66f4e1a51053ba6d65a1a0
c52a357da7a644b8070a151b0',
/ recipients / [
[
/ protected / h'a1013818' / {
\ alg \ 1:-25 \ ECDH-ES + HKDF-256 \
} / ,
/ unprotected / {
/ ephemeral / -1:{
/ kty / 1:2,
/ crv / -1:1,
/ x / -2:h'98f50a4ff6c05861c8860d13a638ea56c3f5ad7590bbf
bf054e1c7b4d91d6280',
/ y / -3:true
},
/ kid / 4:'meriadoc.brandybuck@buckland.example'
},
/ ciphertext / h''
]
]
]
)
C.3.2. Direct plus Key Derivation
This example uses the following:
o CEK: AES-CCM w/128-bit key, truncate the tag to 64 bits
o Recipient class: Use HKDF on a shared secret with the following
implicit fields as part of the context.
* salt: "aabbccddeeffgghh"
* APU identity: "lighting-client"
* APV identity: "lighting-server"
* Supplementary Public Other: "Encryption Example 02"
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Size of binary file is 91 bytes
96(
[
/ protected / h'a1010a' / {
\ alg \ 1:10 \ AES-CCM-16-64-128 \
} / ,
/ unprotected / {
/ iv / 5:h'89f52f65a1c580933b5261a76c'
},
/ ciphertext / h'753548a19b1307084ca7b2056924ed95f2e3b17006dfe93
1b687b847',
/ recipients / [
[
/ protected / h'a10129' / {
\ alg \ 1:-10
} / ,
/ unprotected / {
/ salt / -20:'aabbccddeeffgghh',
/ kid / 4:'our-secret'
},
/ ciphertext / h''
]
]
]
)
C.3.3. Counter Signature on Encrypted Content
This example uses the following:
o CEK: AES-GCM w/ 128-bit key
o Recipient class: ECDH Ephemeral-Static, Curve P-256
Size of binary file is 326 bytes
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[
/ protected / h'a10101' / {
\ alg \ 1:1 \ AES-GCM 128 \
} / ,
/ unprotected / {
/ iv / 5:h'c9cf4df2fe6c632bf7886413',
/ countersign / 7:[
/ protected / h'a1013823' / {
\ alg \ 1:-36
} / ,
/ unprotected / {
/ kid / 4:'bilbo.baggins@hobbiton.example'
},
/ signature / h'00929663c8789bb28177ae28467e66377da12302d7f9
594d2999afa5dfa531294f8896f2b6cdf1740014f4c7f1a358e3a6cf57f4ed6fb02f
cf8f7aa989f5dfd07f0700a3a7d8f3c604ba70fa9411bd10c2591b483e1d2c31de00
3183e434d8fba18f17a4c7e3dfa003ac1cf3d30d44d2533c4989d3ac38c38b71481c
c3430c9d65e7ddff'
]
},
/ ciphertext / h'7adbe2709ca818fb415f1e5df66f4e1a51053ba6d65a1a0
c52a357da7a644b8070a151b0',
/ recipients / [
[
/ protected / h'a1013818' / {
\ alg \ 1:-25 \ ECDH-ES + HKDF-256 \
} / ,
/ unprotected / {
/ ephemeral / -1:{
/ kty / 1:2,
/ crv / -1:1,
/ x / -2:h'98f50a4ff6c05861c8860d13a638ea56c3f5ad7590bbf
bf054e1c7b4d91d6280',
/ y / -3:true
},
/ kid / 4:'meriadoc.brandybuck@buckland.example'
},
/ ciphertext / h''
]
]
]
)
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C.3.4. Encrypted Content with External Data
This example uses the following:
o CEK: AES-GCM w/ 128-bit key
o Recipient class: ECDH static-Static, Curve P-256 with AES Key Wrap
o Externally Supplied AAD: h'0011bbcc22dd44ee55ff660077'
Size of binary file is 173 bytes
96(
[
/ protected / h'a10101' / {
\ alg \ 1:1 \ AES-GCM 128 \
} / ,
/ unprotected / {
/ iv / 5:h'02d1f7e6f26c43d4868d87ce'
},
/ ciphertext / h'64f84d913ba60a76070a9a48f26e97e863e28529d8f5335
e5f0165eee976b4a5f6c6f09d',
/ recipients / [
[
/ protected / h'a101381f' / {
\ alg \ 1:-32 \ ECHD-SS+A128KW \
} / ,
/ unprotected / {
/ static kid / -3:'peregrin.took@tuckborough.example',
/ kid / 4:'meriadoc.brandybuck@buckland.example',
/ U nonce / -22:h'0101'
},
/ ciphertext / h'41e0d76f579dbd0d936a662d54d8582037de2e366fd
e1c62'
]
]
]
)
C.4. Examples of Encrypted Messages
C.4.1. Simple Encrypted Message
This example uses the following:
o CEK: AES-CCM w/ 128-bit key and a 64-bit tag
Size of binary file is 52 bytes
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[
/ protected / h'a1010a' / {
\ alg \ 1:10 \ AES-CCM-16-64-128 \
} / ,
/ unprotected / {
/ iv / 5:h'89f52f65a1c580933b5261a78c'
},
/ ciphertext / h'5974e1b99a3a4cc09a659aa2e9e7fff161d38ce7edd5617
388e77baf'
]
)
C.4.2. Encrypted Message w/ a Partial IV
This example uses the following:
o CEK: AES-CCM w/ 128-bit key and a 64-bit tag
o Prefix for IV is 89F52F65A1C580933B52
Size of binary file is 41 bytes
16(
[
/ protected / h'a1010a' / {
\ alg \ 1:10 \ AES-CCM-16-64-128 \
} / ,
/ unprotected / {
/ partial iv / 6:h'61a7'
},
/ ciphertext / h'252a8911d465c125b6764739700f0141ed09192da5c69e5
33abf852b'
]
)
C.5. Examples of MACed messages
C.5.1. Shared Secret Direct MAC
This example uses the following:
o MAC: AES-CMAC, 256-bit key, truncated to 64 bits
o Recipient class: direct shared secret
Size of binary file is 57 bytes
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[
/ protected / h'a1010f' / {
\ alg \ 1:15 \ AES-CBC-MAC-256//64 \
} / ,
/ unprotected / {},
/ payload / 'This is the content.',
/ tag / h'9e1226ba1f81b848',
/ recipients / [
[
/ protected / h'',
/ unprotected / {
/ alg / 1:-6 / direct /,
/ kid / 4:'our-secret'
},
/ ciphertext / h''
]
]
]
)
C.5.2. ECDH Direct MAC
This example uses the following:
o MAC: HMAC w/SHA-256, 256-bit key
o Recipient class: ECDH key agreement, two static keys, HKDF w/
context structure
Size of binary file is 214 bytes
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[
/ protected / h'a10105' / {
\ alg \ 1:5 \ HMAC 256//256 \
} / ,
/ unprotected / {},
/ payload / 'This is the content.',
/ tag / h'81a03448acd3d305376eaa11fb3fe416a955be2cbe7ec96f012c99
4bc3f16a41',
/ recipients / [
[
/ protected / h'a101381a' / {
\ alg \ 1:-27 \ ECDH-SS + HKDF-256 \
} / ,
/ unprotected / {
/ static kid / -3:'peregrin.took@tuckborough.example',
/ kid / 4:'meriadoc.brandybuck@buckland.example',
/ U nonce / -22:h'4d8553e7e74f3c6a3a9dd3ef286a8195cbf8a23d
19558ccfec7d34b824f42d92bd06bd2c7f0271f0214e141fb779ae2856abf585a583
68b017e7f2a9e5ce4db5'
},
/ ciphertext / h''
]
]
]
)
C.5.3. Wrapped MAC
This example uses the following:
o MAC: AES-MAC, 128-bit key, truncated to 64 bits
o Recipient class: AES keywrap w/ a pre-shared 256-bit key
Size of binary file is 109 bytes
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[
/ protected / h'a1010e' / {
\ alg \ 1:14 \ AES-CBC-MAC-128//64 \
} / ,
/ unprotected / {},
/ payload / 'This is the content.',
/ tag / h'36f5afaf0bab5d43',
/ recipients / [
[
/ protected / h'',
/ unprotected / {
/ alg / 1:-5 / A256KW /,
/ kid / 4:'018c0ae5-4d9b-471b-bfd6-eef314bc7037'
},
/ ciphertext / h'711ab0dc2fc4585dce27effa6781c8093eba906f227
b6eb0'
]
]
]
)
C.5.4. Multi-recipient MACed message
This example uses the following:
o MAC: HMAC w/ SHA-256, 128-bit key
o Recipient class: Uses three different methods
1. ECDH Ephemeral-Static, Curve P-521, AES-Key Wrap w/ 128-bit
key
2. AES-Key Wrap w/ 256-bit key
Size of binary file is 309 bytes
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[
/ protected / h'a10105' / {
\ alg \ 1:5 \ HMAC 256//256 \
} / ,
/ unprotected / {},
/ payload / 'This is the content.',
/ tag / h'bf48235e809b5c42e995f2b7d5fa13620e7ed834e337f6aa43df16
1e49e9323e',
/ recipients / [
[
/ protected / h'a101381c' / {
\ alg \ 1:-29 \ ECHD-ES+A128KW \
} / ,
/ unprotected / {
/ ephemeral / -1:{
/ kty / 1:2,
/ crv / -1:3,
/ x / -2:h'0043b12669acac3fd27898ffba0bcd2e6c366d53bc4db
71f909a759304acfb5e18cdc7ba0b13ff8c7636271a6924b1ac63c02688075b55ef2
d613574e7dc242f79c3',
/ y / -3:true
},
/ kid / 4:'bilbo.baggins@hobbiton.example'
},
/ ciphertext / h'339bc4f79984cdc6b3e6ce5f315a4c7d2b0ac466fce
a69e8c07dfbca5bb1f661bc5f8e0df9e3eff5'
],
[
/ protected / h'',
/ unprotected / {
/ alg / 1:-5 / A256KW /,
/ kid / 4:'018c0ae5-4d9b-471b-bfd6-eef314bc7037'
},
/ ciphertext / h'0b2c7cfce04e98276342d6476a7723c090dfdd15f9a
518e7736549e998370695e6d6a83b4ae507bb'
]
]
]
)
C.6. Examples of MAC0 messages
C.6.1. Shared Secret Direct MAC
This example uses the following:
o MAC: AES-CMAC, 256-bit key, truncated to 64 bits
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o Recipient class: direct shared secret
Size of binary file is 37 bytes
17(
[
/ protected / h'a1010f' / {
\ alg \ 1:15 \ AES-CBC-MAC-256//64 \
} / ,
/ unprotected / {},
/ payload / 'This is the content.',
/ tag / h'726043745027214f'
]
)
Note that this example uses the same inputs as Appendix C.5.1.
C.7. COSE Keys
C.7.1. Public Keys
This is an example of a COSE Key set. This example includes the
public keys for all of the previous examples.
In order the keys are:
o An EC key with a kid of "meriadoc.brandybuck@buckland.example"
o An EC key with a kid of "peregrin.took@tuckborough.example"
o An EC key with a kid of "bilbo.baggins@hobbiton.example"
o An EC key with a kid of "11"
Size of binary file is 481 bytes
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[
{
-1:1,
-2:h'65eda5a12577c2bae829437fe338701a10aaa375e1bb5b5de108de439c0
8551d',
-3:h'1e52ed75701163f7f9e40ddf9f341b3dc9ba860af7e0ca7ca7e9eecd008
4d19c',
1:2,
2:'meriadoc.brandybuck@buckland.example'
},
{
-1:1,
-2:h'bac5b11cad8f99f9c72b05cf4b9e26d244dc189f745228255a219a86d6a
09eff',
-3:h'20138bf82dc1b6d562be0fa54ab7804a3a64b6d72ccfed6b6fb6ed28bbf
c117e',
1:2,
2:'11'
},
{
-1:3,
-2:h'0072992cb3ac08ecf3e5c63dedec0d51a8c1f79ef2f82f94f3c737bf5de
7986671eac625fe8257bbd0394644caaa3aaf8f27a4585fbbcad0f2457620085e5c8
f42ad',
-3:h'01dca6947bce88bc5790485ac97427342bc35f887d86d65a089377e247e
60baa55e4e8501e2ada5724ac51d6909008033ebc10ac999b9d7f5cc2519f3fe1ea1
d9475',
1:2,
2:'bilbo.baggins@hobbiton.example'
},
{
-1:1,
-2:h'98f50a4ff6c05861c8860d13a638ea56c3f5ad7590bbfbf054e1c7b4d91
d6280',
-3:h'f01400b089867804b8e9fc96c3932161f1934f4223069170d924b7e03bf
822bb',
1:2,
2:'peregrin.took@tuckborough.example'
}
]
C.7.2. Private Keys
This is an example of a COSE Key set. This example includes the
private keys for all of the previous examples.
In order the keys are:
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o An EC key with a kid of "meriadoc.brandybuck@buckland.example"
o A shared-secret key with a kid of "our-secret"
o An EC key with a kid of "peregrin.took@tuckborough.example"
o A shared-secret key with a kid of "018c0ae5-4d9b-471b-
bfd6-eef314bc7037"
o An EC key with a kid of "bilbo.baggins@hobbiton.example"
o An EC key with a kid of "11"
Size of binary file is 816 bytes
[
{
1:2,
2:'meriadoc.brandybuck@buckland.example',
-1:1,
-2:h'65eda5a12577c2bae829437fe338701a10aaa375e1bb5b5de108de439c0
8551d',
-3:h'1e52ed75701163f7f9e40ddf9f341b3dc9ba860af7e0ca7ca7e9eecd008
4d19c',
-4:h'aff907c99f9ad3aae6c4cdf21122bce2bd68b5283e6907154ad911840fa
208cf'
},
{
1:2,
2:'11',
-1:1,
-2:h'bac5b11cad8f99f9c72b05cf4b9e26d244dc189f745228255a219a86d6a
09eff',
-3:h'20138bf82dc1b6d562be0fa54ab7804a3a64b6d72ccfed6b6fb6ed28bbf
c117e',
-4:h'57c92077664146e876760c9520d054aa93c3afb04e306705db609030850
7b4d3'
},
{
1:2,
2:'bilbo.baggins@hobbiton.example',
-1:3,
-2:h'0072992cb3ac08ecf3e5c63dedec0d51a8c1f79ef2f82f94f3c737bf5de
7986671eac625fe8257bbd0394644caaa3aaf8f27a4585fbbcad0f2457620085e5c8
f42ad',
-3:h'01dca6947bce88bc5790485ac97427342bc35f887d86d65a089377e247e
60baa55e4e8501e2ada5724ac51d6909008033ebc10ac999b9d7f5cc2519f3fe1ea1
d9475',
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-4:h'00085138ddabf5ca975f5860f91a08e91d6d5f9a76ad4018766a476680b
55cd339e8ab6c72b5facdb2a2a50ac25bd086647dd3e2e6e99e84ca2c3609fdf177f
eb26d'
},
{
1:4,
2:'our-secret',
-1:h'849b57219dae48de646d07dbb533566e976686457c1491be3a76dcea6c4
27188'
},
{
1:2,
-1:1,
2:'peregrin.took@tuckborough.example',
-2:h'98f50a4ff6c05861c8860d13a638ea56c3f5ad7590bbfbf054e1c7b4d91
d6280',
-3:h'f01400b089867804b8e9fc96c3932161f1934f4223069170d924b7e03bf
822bb',
-4:h'02d1f7e6f26c43d4868d87ceb2353161740aacf1f7163647984b522a848
df1c3'
},
{
1:4,
2:'our-secret2',
-1:h'849b5786457c1491be3a76dcea6c4271'
},
{
1:4,
2:'018c0ae5-4d9b-471b-bfd6-eef314bc7037',
-1:h'849b57219dae48de646d07dbb533566e976686457c1491be3a76dcea6c4
27188'
}
]
Acknowledgments
This document is a product of the COSE working group of the IETF.
The following individuals are to blame for getting me started on this
project in the first place: Richard Barnes, Matt Miller, and Martin
Thomson.
The initial version of the draft was based to some degree on the
outputs of the JOSE and S/MIME working groups.
The following individuals provided input into the final form of the
document: Carsten Bormann, John Bradley, Brain Campbell, Michael B.
Schaad Expires May 26, 2017 [Page 120]
Internet-Draft CBOR Object Signing and Encryption (COSE) November 2016
Jones, Ilari Liusvaara, Francesca Palombini, Goran Selander, and
Ludwig Seitz.
Author's Address
Jim Schaad
August Cellars
Email: ietf@augustcellars.com
Schaad Expires May 26, 2017 [Page 121]
